We're talking like, .5% of games, here. Those games are backed by Microsoft studios, too. Shadowrun, Halo 2, and Alan Wake are the three that I was thinking of. Not sure if there are any others, but I know those were the larger of the games being pushed for DX10. ...especially Alan Wake.

Almost positive it is impossible to put DX10 on XP - though why that is I can't remember at the moment. Something to do with the backbone of it I think...