I'm about to make a new build. I'm looking to play Crysis Warhead, Fallout 3 and Left 4 Dead, plus any new games that are coming out soon. I play at 1920 x 1080 res, will a GTX280 be enough?
Your CPU is the next most important factor. the higher you can get your clockspeed the better. Tom's did an article about the importance of upgrading your cpu vs. gpu and found that while your gfx card was the most vital thing, the actual clockspeed of your CPU came in at a reasonably close second. Ive got an e7400 at 4GHz and an overclocked 4850. Those two get me a stable 55fps in Fallout 3 with ultra quality settings at 1920x1080. and appx 35 FPS in Warhead both w/o AA. Left 4 Dead isnt an issue. It runs well on almost anything, cuz Valve's source engine is hoss.
You're not going to get more than 30 fps with gtx 280 with everything up on crysis.
Also, I hope you plan on buying a good power supply, which some people over look.