i don't think any specific brand will work better. It's just marketing strategy i'd say, Blizzard gets money from ati and ati gets advertising saying this game works well with their product. As long as you've got a descent video card it will run fine no matter what the brand.
yeah, most likely, blizzard wouldn't make it so their game only runs at it's best on a specific brand. granted some brands technology may be better than the other at the moment, but two cards with the same specs should run it at the same quality. 512 mb is still a mid to top range video card so you should be good. i'm building a new pc and putting this card in it.
I don't know much about ATI and Nvidia and drivers and such things, so take what I say with a grain of salt.
But to me it makes sense to optimize the game to run equally well on both brands. having significant differences in terms of game performance on either brand would piss off the people that don't have the card from the manufacturer who is in the sack with Blizzard, if I could make an analogy !
Uhm.... D3 is using the same engine as Oblivion right? Then iirc ATI should have an advantage over most other cards. But this is all theory, I for one never notice much of a difference between brands when the graphics cards have the same specs.
It's a proprietary engine which will incorporate the Havok physics engine.
The bottom line is, Diablo 3 is not going to favour nvidia or ATI in any way as far as performance is concerned, it comes down to the individual card specifications and to run Diablo 3 at playable settings old cards like 6200GT and the such will probably still be alright.
tldr: You won't need a beast of a graphics card to play Diablo 3.