From the readme file:
Minimum System Requirements
R4 may well run on a computer less powerful than this, but i will not listen to any bug reports if your computer doesn't meet the following spec:
NVidia GeForce 3 or better (and the LATEST drivers). ATI cards may work with minor artefacts.
1Ghz CPU
256mb RAM
From what I can tell it doesn't really take much by todays standards. I can't really guess what your performance will be, but with any of todays dedicated graphics cards, I would expect fairly good performance. I'm running it on a 8800M GTX. With it running wide open and not Vsync'd, it can push 300+ FPS. I keep the vsync on though so it doesn't flicker by changing a frame in mid draw on screen. Even on a 4 year old 2GHz proc lappy with no dedicated graphics card I could do 20'ish FPS, so I wouldn't worry.
