The 6600GT would be a significant upgrade over the 9600XT, but not over an existing 9800XT, where a 6800GT would be a more sensible, albeit more expensive choice. The ATI Radeon 9800XT is still a fairly powerful video card, even by today's standards. So much so, that they were hastily removed from the marketplace, (if anyone noticed the sudden disappearance!), in order to make the newer generation cards look more attractive, and also hide the fact that the new cards, in many cases, do not perform as good as the old 9800XT's do.
I'll also add that the ATI 9800XT was a premium priced card right up to it's untimely demise, suggesting a strong demand for it, supporting my conspiracy theory.


Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&