Its been a long time since I played Diablo II...
but Diablo II is an ancient game, and was designed on the 3dfx Glide API, so the Voodoo cards will get the most out of Diablo II using HW acceleration. Also note that this game is highly CPU intensive (ironic ... ) because it uses NO 3D whatsoever; just some lighting effects. So basically I believe the only thing that would give you better performance is more CPU power or putting that Voodoo back in.