Here's a comparrison of the xpress v's X700 (ati's competitor to the 6600 series.
http://3dgpu.com/archives/2004/11/08/ati-radeon-xpress-200g-review/4/
Doom 3 1024x768 medium quality
On a budget I'd stick a 6600GT in there which can be gotten for about $140 that should at least double the performance of the 9600XT in most cases
Don't get a non 6600GT as you lose about half your performance.
http://www.amdzone.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=93&page=2
http://3dgpu.com/archives/2004/11/08/ati-radeon-xpress-200g-review/4/
Doom 3 1024x768 medium quality
On a budget I'd stick a 6600GT in there which can be gotten for about $140 that should at least double the performance of the 9600XT in most cases
Don't get a non 6600GT as you lose about half your performance.
http://www.amdzone.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=93&page=2