Well, although Dx10 is obviously better than Dx9 in so many ways, all those graphics will be useless if the game design is poor. Pretty stuff is only useful if it does enhance the gameplay factor and contributes to the environment(not just slapping in AAF and stuff). Otherwise, it's just another way to tout a huge budget so you can draw in the graphics whores.
Btw, Vista is just as functional as XP, albeit with a bit of speed boost and as it is, I don't see any motivations for me to upgrade. Besides, isn't Vienna coming out in 2 to 3 years' time?