I haven't bought Diablo, rather I chose instead to put my limited gaming $$ into Ice Wind Dale. A close friend bought both. We are doing IWD multiplayer and he does D2 in between. He is an avid RPG gamer and a elementary school teacher in in central NY. He is well read w/respect to mideaval mythology and literature, hence the draw to RPG games... He feels D2 is fun to play, but is disappointed with it's lack of depth and story (i.e. mostly hack and slash), which makes him feel the game is aimed primarily at teenagers. My friend is around my age (~40). But I digress...
My friend has IIRC a 8 meg video card and has no problem running D2 on his machine. It is a 400mhz PII w/64 meg ram. Although even he recognizes that he will need to upgrade to a minimum Voodoo 3 card this fall with new games coming out like Never Winter Nights & Pool of Radiance. With all the high power and expensive video cards coming out, the Voodoo 3 cards will be a bargain. It is still powerful and probably has the least incompatibility problems with games. So if you don't have to have all the latest wistles and bells, esp 32 bit 3D color, anti aliasing (which hurts frame rate performance anyway) and 2048x2048 textures etc... then you don't need to spend megabucks ~$250 on a new card. I'm finding my Voodoo 3 to be a very servicable card and a good investment. Eventually, I will replace it, but probably not for at least 6 months or more.
Good luck
Jim