Well what helps with having high resolutions is the number of pixel pipelines in your card. Having more pipelines allows you to run high resolutions while maintaining fast framerates, the 6600GT has 8 which isn't bad but with the better cards having 12 or 16 and so many demanding games coming out, my guess is you won't be able to run 1024x768 high for more than 1 year, maybe 1.5 years.
Yea, according to 3DMark05, my 9800XT (a little slower than 6600) will run future games (in 18 months) as a slideshow! Yay! You'd be better off with a 6800GT or waiting for the new generation of vid cards to come out if yours isnt too bad.
cs 1.6? cs 1.6 is not hardware demanding at all, my friend plays it 1024x with motherboard video. cs source is not THAT hardware demanding either. if you have AA and AF off. 6600GT will boost performence tremendously, and you can most likely play with full AA/AF.
id say 1 year with high settings, 2 years medium, and after 3 years, well you should have upgraded by then.