Theres no need for 64 bit on a game this small. I only saw about a gig of RAM/VRAM being used by this game on max settings.
On topic, Ive got an AMD processor, but an NVIDIA card and it works very well. I remember friends having issues with ATI/AMD cards and Unreal Engine, so its probably an issue with the card.
On a side note, updated drivers can often provide a boost to game performance.