Judging from past games and blizzard's motives, I think any mid range performing graphics card will do perfectly fine.
Their mindset seems to be with getting the best looking settings on a mid-high range card settings, anything over is nice but overkill. It probably has to do with reasons for marketing to the right specs of people's PC's too.
Believe it or not, I'm sure a majority of gamers out there (normal, not enthusiasts makes up a great majority) would not like the fact of having to upgrade to be able to play D3/S2.