Well, I pondered upgrading to an Nvidia DX11 card to get the full Phsyx and all. I guess now I am glad that I did not. It seems most complaints are related to DX11. I have not had a single issue playing this game with my machine.
My machine is as follows: Windows 7 Professional 64 bit Core i7 965 3.2 GHz (Quad Core) ATI Radeon HD4870X2 (directx 10.1 card) 8 GB Ram
And I must say that this is an absolutely beautiful game. I was able to max all settings, except the lack of Physx and the 1920X1080 limitation of my monitor. It is a gorgeous game visually even without DX11 and Physx. The voice acting, just like on Arkham Asylum (which I also have on pc and never had a single issue with)is amazing.
The plot and story of the game also is incredible. I have finished all but Catwoman's last mission or 2. I thought that the story line was better than any Batman story I have read/watched/ or played in the past (not to imply I am a Batman junkie. I rarely read comics, but LOVE the Christopher Nolan movies, as well as the old Animated Series). So many twists and turns. Just amazing writing.
Anyways. If you have a DirectX 10.1 card, based on my experience, you should have no problems; go ahead and buy, and prepare to be amazed. DirectX 11, I cannot speak for and I would trust the other reviews on that. I have seen this complaint on the AC forums as well; and I do really hate it when I get stuck with issues like that and I do not doubt their frustrations. Sorry guys.
BTW: The Catwoman content is already present on the PC version. No need to download at all!