CoD4, to me, seems like CoD2 and CoD3 with Arabs and modern guns. Graphics pretty much look the same with some razzle-dazzle here and there. I will give the game one thing though, it's got the best night-vision effect I've ever seen. Can't say I've played the multiplayer, so I wouldn't know about that.
However, I'm still gonna press Crysis, but then again, CoD4 will run a lot better on your system. Conclusion: Get CoD4 now, and Crysis once you upgrade that computer (sell it and build a new one).