So I've upgraded to Vista 32 today
I can now compare the 2 OSes, one with GSdx9, the other with GSdx10 on a Geforce 8800GTS.
The overall image quality is greatly improved with GSdx10, all those small Z-Buffer bugs are completely gone.
Also the framerate is much more consistant, games with heavy slowdown under GSdx9 now run near full speed.
Xenosaga especially stands out as it has a 3-4 times higher framerate
Overall the Vista overhead takes some 5% speed from games that already run well with GSdx9.
These games are also faster with WinXp, if only a few fps.
So you did a terrific job at improving the already good GSdx9, Gabest.
Games are really way more enjoyable with GSdx10, thank you!