I would go for any 2x2GB combination that is offered by any of the big brands. Talking from what experience I have with a similar setup that I built last week for myself.
These are my specs:
Core i5 750
2x2GB Transcend DDR3-1333
Have had no problems with playing any games, even with only one RAM stick in. Any decent RAM combination should be able to play Left4Dead no problem.
Mine plays TF2 @ 300FPS (wont go faster because I haven't turned the frame limiter off).