Hi

I would say postphoning will be a wise decision. It'll save u money and moreover with the Microsoft Vista being launched, upgrading ur PC accordingly will be a good decision.

Vista will have DirectX 10 support which is not in XP and newer games will require DirextX 10 compliant Graphic cards.

See the link !
http://www.theinquirer.net/default.aspx?article=32444

Hope it helps !