Yes DX10 is for vista only.
I nearly went for Vista 64 when I upgraded, but then sent it back after hearing that BF2 doesn't work with it.
But after the LANFEST I've changed my mind, 2 peeps had vista (32) on their PC's and all the games ran perfectly. And both said they have had no problems at all with the OS.
So once I've got the £56 spare I'll probably buy it. though I will keep XP on the other HDD.