A GTX 560 will be good (its' what I have. Well, the Asus Factory OC version, anyway). In any case, even a 560 OC won't do max settings at 1080p in a game like Crysis or GTA IV. I would suggest at least a 560 Ti or 570 if max quality at 1080p is your goal. If you're using a lower res, then a 560 will be just fine.
Regardless, the GTS 450 won't get you good FPS at High/Max quality at any decent res (maybe 720p).