What reason do you feel the need to upgrade to vista? DX10 has not been released, nor has SP1. Driver support for Vista is still spotty. If you dont have any dire need to upgrade to vista, hold off until SP1 or DX10 is released. If you are itching for the graphic features then you have to determine if it is worth the price tag.
Check out this thread for more vista thoughts:
"Upgraded" from Vista to XP