Sounds like SP1 for Vista will make things run better and looks like it will have Direct X 10 support and SP 3 for XP will be out as well 1st part of next year. So wait for the Direct X 10 cards to drop in price some best time to buy will be next year.
Microsoft Windows Vista SP1 Beta: Full Review - Review by PC Magazine
Microsoft Windows Vista SP1 Beta: Full Review - Review by PC Magazine