Those specs will run anything using the Source engine; though you'll notice lower performance on anything post-Orange Box (that is, TF2, Portal, Half-Life Episode 2). CS S and HLDM will run ok.
2 gigs of mem is worth getting, for sure.
As for running it in 640x480: I would recommend that you allow the game to choose the video settings for you. Probably 1024x768, I'd wager. It will look much better and you probably won't see a much of an increase in frame rate with the lower res, so you might as well.
Edit: I actually have loaded CS S on one of my older machines: P3 800 mHz, 512 ram, and a GForce 4 64 meg video card. Very low settings...but was able to get a respectable 45 fps in the Source stress test. Not pretty...but playable!
Edited by locally pwned, 12 March 2009 - 09:52 AM.
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