http://www.overclockers.co.uk/acatal...00_Series.html
The 9700Pro would make a great budget upgrade, about 2-3 times the performance you have now, and will play most games pretty damn good, way better than what u have now.
A 9800 Pro is a nice step up, but realy bumps up the cost (DON'T get the SE version, unless u are prepared to mod it )
Of course the X-800 series is the best for ATI, but is pretty expensive.
NVIDIA, go with a 6600GT, or any of the 6800's if you have the big bucks, just make sure you get the AGP version, not the PCI express!