GTX 275 vs GTX 275 SLI

[GamingNerd]

I am making a computer at the moment and I am looking for a good graphics card (or multiple).I am at the price range of about 600 or so on GPU's. I thought of getting a GTX 275 because it looked good and heard good reviews about it. I was wondering how big of a difference there is between a GTX 275 and a SLI'd GTX 275. Also for the price of 600, is there anything better I can get?
Monitor: 22" Samsung (Might upgrade in the future). Thanks in advance!
EDIT:: Currently I am only going to play Counter Strike Source and such. I was also wondering if there was anything better than the GTX 275 SLi'd.

 

[masterjaw]

There's no reason to go SLI just to play with a 22" monitor. A single GTX 275 is enough.

 

[jiro]

just buy 1 gtx275 and see the results.
if you feel that it isnt enough, buy another one and sli it.