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                                                                     Exhibit 8.1
                                 April 15, 1999


3Dfx Interactive, Inc.
4435 Fortran Drive
San Jose, California, 95134

Ladies and Gentlemen:

     This opinion is being delivered to you in connection with the Form S-4
Registration Statement filed with the Securities and Exchange Commission (which
contains a Joint Proxy Statement/Prospectus) (the "Registration Statement")
filed pursuant to the Agreement and Plan of Reorganization (the "Reorganization
Agreement"), dated December 13, 1998, among 3Dfx Interactive, Inc., a California
corporation ("3Dfx"), Voodoo Merger Sub, Inc., a Texas corporation and a
wholly-owned subsidiary of 3Dfx ("Merger Sub"), and STB Systems, Inc., a Texas
corporation ("STB").

     Except as otherwise provided, capitalized terms used but not defined herein
shall have the meanings set forth in the Reorganization Agreement.

     We have acted as counsel to 3Dfx and Merger Sub in connection with the
Merger. As such, and for the purpose of rendering this opinion, we have
examined, and are relying upon (without any independent investigation or review
thereof) the truth and accuracy, at all relevant times, of the statements,
covenants, representations and warranties contained in the following documents
(including all exhibits and schedules attached thereto):

     1.   The Reorganization Agreement;

     2.   The Form S-4 Registration Statement filed with the Securities and
          Exchange Commission on April 15, 1999 (which contains a Joint Proxy
          Statement/Prospectus);

     3.   Those certain tax representation letters expected to be delivered to
          us by 3Dfx, Merger Sub and STB containing certain representations of
          3Dfx, Merger Sub and STB (the "Tax Representation Letters"); and

     4.   Such other instruments and documents related to the formation,
          organization and operation of 3Dfx, Merger Sub and STB and related to
          the consummation of the Merger and the other transactions contemplated
          by the Reorganization Agreement as we have deemed necessary or
          appropriate.

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3Dfx Interactive, Inc.
April 15, 1999
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     In connection with rendering this opinion, we have assumed (without any
independent investigation or review thereof) that:

     1.   Original documents submitted to us (including signatures thereto) are
          authentic, documents submitted to us as copies conform to the original
          documents, and that all such documents have been (or will be by the
          Effective Time) duly and validly executed and delivered where due
          execution and delivery are a prerequisite to the effectiveness
          thereof;

     2.   All representations, warranties and statements made or agreed to by
          3Dfx, Merger Sub and STB, their managements, employees, officers,
          directors and shareholders in connection with the Merger, including,
          but not limited to, those set forth in the Reorganization Agreement
          (including the exhibits thereto) and the Tax Representation Letters
          are true and accurate at all relevant times;

     3.   All covenants contained in the Reorganization Agreement (including
          exhibits thereto) and the Tax Representation Letters are performed
          without waiver or breach of any material provision thereof;

     4.   The Merger will be reported by 3Dfx, Merger Sub and STB on their
          respective federal income tax returns in a manner consistent with the
          opinion set forth below; and

     5.   Any representation or statement made "to the best of knowledge" or
          similarly qualified is correct without such qualification.

     Based on our examination of the foregoing items and subject to the
limitations, qualifications, assumptions and caveats set forth herein, we are of
the opinion that:

     1.   For United States federal income tax purposes, the Merger will be a
          "reorganization" within the meaning of Section 368(a) of the Internal
          Revenue Code of 1986, as amended;

     2.   No gain or loss will be recognized by holders of STB common stock
          solely upon their receipt of 3Dfx common stock solely in exchange for
          STB common stock in the merger, except to the extent of cash received
          in lieu of a fractional share of 3Dfx common stock;

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3Dfx Interactive, Inc.
April 15, 1999
Page 3


     3.   The aggregate tax basis of 3Dfx common stock received by STB
          shareholders in the merger will be the same as the aggregate tax basis
          of STB common stock surrendered in exchange therefor, reduced by any
          amount allocable to a fractional share interest for which cash is
          received;

     4.   The holding period of 3Dfx common stock received in the merger will
          include the period for which the STB common stock surrendered in
          exchange therefor was held, provided that the STB common stock is held
          as a capital asset at the time of the merger;

     5.   Cash payments received by holders of STB common stock in lieu of a
          fractional share will be treated as if a fractional share of 3Dfx
          common stock had been issued in the merger and then redeemed by 3Dfx.
          A shareholder of STB receiving such cash will generally recognize gain
          or loss upon such payment, equal to the difference, if any, between
          such shareholder's allocable basis in the fractional share and the
          amount of cash received. This gain or loss will be a capital gain or
          loss if the STB common stock is held by such shareholder as a capital
          asset at the effective time of the merger; and

     6.   Neither 3Dfx, Merger Sub nor STB will recognize gain or loss solely as
          a result of the merger.

     This opinion does not address the various state, local or foreign tax
consequences that may result from the Merger or the other transactions
contemplated by the Reorganization Agreement. In addition, no opinion is
expressed as to any tax consequence of the Merger or the other transactions
contemplated by the Reorganization Agreement except as specifically set forth
herein, and this opinion may not be relied upon except with respect to the
consequences specifically discussed herein.

     No opinion is expressed (i) as to any transaction other than the Merger as
described in the Reorganization Agreement, or (ii) as to any other transaction
whatsoever, including the Merger, if all of the transactions described in the
Reorganization Agreement are not consummated in accordance with the terms of the
Reorganization Agreement and without waiver of any material provision thereof.
To the extent that any of the representations, warranties, covenants, statements
and assumptions material to our opinion and upon which we have relied are not
accurate and complete in all material respects at all relevant times, our
opinion would be adversely affected and should not be relied upon.

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3Dfx Interactive, Inc.
April 15, 1999
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     This opinion only represents our best judgment as to the federal income tax
consequences of the Merger and is not binding on the Internal Revenue Service or
any court of law, tribunal, administrative agency or other governmental body.
The conclusions are based on the Internal Revenue Code of 1986, as amended,
existing judicial decisions, administrative regulations and published rulings.
No assurance can be given that future legislative, judicial or administrative
changes or interpretations would not adversely affect the accuracy of the
conclusions stated herein. Nevertheless, by rendering this opinion, we undertake
no responsibility to advise you of any new developments in the application or
interpretation of the federal income tax laws.

     This opinion is being delivered solely for the purposes of complying with
applicable securities laws. It may not be relied upon or utilized for any other
purpose or by any other person and may not be made available to any other person
without our prior written consent. We hereby consent to the use of our name
under the headings "The Merger and Related Transactions - Material Federal
Income Tax Matters" and "Legal Matters" in the Joint Proxy Statement/Pros
pectus+ and to the filing of this opinion as an exhibit to the Registration
Statement.

                                       Very truly yours,


                                       /s/ Wilson Sonsini Goodrich & Rosati
                                       ------------------------------------
                                       WILSON SONSINI GOODRICH & ROSATI
                                       Professional Corporation
