Generally speaking and assuming no overclocking...
550w+ PSU = HD 5870
Great for playing at 1920 x 1200 and probably the best performance for Crysis Warhead with a single video card with a single GPU core. However, setting every graphic setting to max will result in less than 30FPS. Probably will be bottlenecked by your CPU as well.
500w+ PSU = HD 5850
Very good for 1920 x 1200. Glad I bought mine for just $280. Not as fast as the HD 5870 and maxing out every graphic setting to max will probably result in less 26FPS in Crysis Warhead. Might have be slightly bottlenecked by your CPU.
450w+ PSU = HD 5770
Very good for 1680 x 1050. However, maxing out Crysis Warhead will probably give you about 26FPS. Your CPU should not bottleneck this GPU at all. Usable for gaming at 1920 x 1200 as long as you can live with some graphic settings at medium quality from time to time.