I've just installed that new 178.13 driver on my XP SP3 x86 partition and when compared to my previously installed one, namely the 177.83 WHQL, it definitely boosts my performance in Source-based games, in Crysis (not Warhead) and in Kane's Wrath. Already the Counter-Strike: Source's Video Stress Test received a nice little +4 FPS boost, and in Lost Coast I got +6 FPS compared to my previous average (tested by in-game console commands, not FRAPS). I play many Source-based games so I'm happy with the new results.
In Crysis's GPU Benchmark, at 1024x768, I gained a noticeable +5 FPS, and at 1280x960 I got +4 FPS compared to my last tests. And most of us with mid-range systems know that +4 to +6 or so FPS boost in Crysis can certainly make a difference when all you had was 25 FPS previously, so it's now playing at a better 30 FPS in areas that are more demanding than others mostly. It's nothing huge per se, but I will certainly take that free performance boost without changing my GPU or without increasing the over-clock frequencies.
I also noticed that my previously (although rarely) occurring flashlight-related graphical artifacts at night in S.T.A.L.K.E.R.: Clear Sky has been fixed with this new driver. That new driver is certainly a keeper for me. I haven't tested in my Vista x64 partition yet, but that's because only Clear Sky is installed on it for the moment, but I'm sure that it gives a performance boost also in Vista, since according to the posts I've read about the 178.13 at Guru3D's forums it looks like to be the case indeed, the general consensus/observations points towards a better overall driver compared to pretty much all of the previous ones we've had in months.