Over at the Pantheon site, which is chalk full of p clan goodness, they've slapped up a CS tweak guide. Here, of course, is your bit to stare at:
:: Vsync, or not?Don't worry, they get less nerdy!
Vsync basically caps your FPS at your monitors refresh rate. For your average monitor, it will cap it at 60Hz while running at 1280x960, and 85Hz while running at 1024x768. By disabling it, you can get right up there to the 99 mark. There are a few things you need to take into consideration though. Do you really need that extra FPS? Can you even tell the difference between 85 frames and 100? Some people find that with Vsync disabled, their mouse feels as though it is 'laggy'. Some systems won't feel this side effect at all - Try it out and use which suits your needs best.