Stalker Clear Sky : Read Before Patching
If you've bought Stalker Clear Sky and played it some, then by all means, do not update it with any patches from GSC / Deep Silver.
Unlike other major software releases, Stalker CS has a major flaw very early on in the game, and the fix for it makes you have to start over from the beginning by failing to account for fixing your save game files.
So, if you have the store bought version and have played it up until the point where the guide fails to take you to Cordon, you can fix it, but you get to start over completely from scratch.
I like the game, but this is a stretch.