Vista doesn't work well with some games, especially if you are playing online. PunkBuster is a program that controlls/limits cheating and requires administrator access to work. You are not notified, therefore you are kicked off of the server.
And if you don't have DirectX10 Card, it's pointless to upgrade.
If all you do is games, upgrading to Vista now is a huge waste of money. Let Microsoft pick through the problems b4 you buy it.