if you can sell the 9800 pro and get a 6600GT for near the same price, then go for it. Otherwise, it's not worth it. A 6600GT is roughly equivalent to a 9800XT, which is basically a 9800 Pro overclocked. The 6600GT is a bit faster in newer games, due to the newer architecture, although sometimes the 9800XT wins in eye candy modes due to its 256-bit memory interface (although neither card can comfortably run the newest games at 1600x1200 with AA/AF, anyways, so it's kinda a moot point.). Anyways, I wouldn't recommend spending more than $20 on the upgrade. If you can't get it in this range, then I recommend waiting till X800XLs and 6800GTs work their way down to the $200 price point (some X800XLs have actaully been spotted at $275), or for the R520/G70 mainstream part (X750XT/GF 7600GT????), which will likely feature a 16-pipe solution, and maybe even a 256-bit memory interface.
Oh, and the 6600GT cannot be flashed into anything else. It is a native NV43 core, not a cut-down NV40 core, so there are no extra pipelines to unlock like with the 6800. The only thing you can do is overclock, which can actually be quite successful. 10-15% overclocks are pretty common, and 20% isn't out of the question (and a 6600GT at 600/1200 would be pretty damn powerful, with memory bandwidth quite close to that of a 6800NU)
