EXHIBIT 5.1


                                                            (214)740-8675



                                 November 1, 1997


STB Systems, Inc.
1651 North Glenville Drive
Richardson, Texas 75081

    RE:  REGISTRATION OF AN ADDITIONAL 150,000 SHARES OF COMMON STOCK,
         PAR VALUE $.01 PER SHARE, PURSUANT TO A POST-EFFECTIVE AMENDMENT NO. 1
         TO THE COMPANY'S REGISTRATION STATEMENT ON FORM S-8, REGISTRATION 
         NO. 33-89460

Gentlemen:

    We have acted as counsel for STB Systems, Inc., a Texas corporation (the
"Company"), in connection with the registration under the Securities Act of
1933, as amended (the "Securities Act"), pursuant to a Post-Effective Amendment
No. 1 to a Registration Statement on Form S-8 (the "Registration Statement"), of
150,000 shares of common stock, par value $.01 per share, of the Company (the
"Common Stock") to be offered to employees of the Company pursuant to the STB
Systems, Inc. 1995 Long Term Incentive Plan (as amended and restated effective
November 1, 1996) (the "Plan").

    Based upon our examination of such papers and documents as we have deemed
relevant or necessary in rendering this opinion, and based on our review of the
Texas Business Corporation Act, we hereby advise you that we are of the opinion
that, assuming with respect to shares of Common Stock issued after the date
hereof, (i) the receipt of proper consideration for the issuance thereof in
excess of the par value thereof, (ii) the availability of a sufficient number of
shares of Common Stock authorized by the Company's Amended and Restated Articles
of Incorporation then in effect, (iii) compliance with the terms of any
agreement entered into in connection with any options or shares of Common Stock
issued under the Plan, (iv) no change occurs in the applicable law or the
pertinent facts, and (v) the proper grant of options under the Plan, shares of 
Common Stock purchasable under the Plan will be legally issued, fully paid 
and non-assessable shares of Common Stock.

    We consent to the filing of this opinion as Exhibit 5.1 to the Registration
Statement filed by the Company with the Securities and Exchange Commission for
the registration under the 




STB Systems, Inc.
November 1, 1997
Page 2


Securities Act of 150,000 shares of Common Stock covered by the Plan.  By so 
consenting, we do not thereby admit that our firm's consent is required by 
Section 7 of the Securities Act.

                                        Very truly yours,

                                        LOCKE PURNELL RAIN HARRELL
                                        (A Professional Corporation)


                                        By: /s/ JOHN B. MCKNIGHT
                                           -------------------------------
                                           John B. McKnight