Ok, here's what I got mainly from notebookcheck.net
On 3DMark '05, compared to the original MBP SR 2.2GHZ 8600MGT with 120mb Vram, the new 9600M GT is 40% faster.
Compared to the 2.4GHZ MBP SR with 256mb of vram, the difference is 21%.
Compared to the 2.5GHZ MBP Penryn with 512mb of vram, the difference narrows down to a more modest 10%!
Bottom line is that it makes more sense to refresh (ie you will get more bang for the buck) if you are the owner of a 2.2GHZ SR than if you have a 2.5GHZ Penryn