The 9700 was/is based on the R300 ATI core. While being a rather incredible design, it had flaws. The 9800's R350 core increased the performance of vertex shaders, corrected a couple of issues, and increased stability.
Overall performance for the R350 is up due to a new layout design which allows the R350 to clock higher than the R300, while maintaining the power draw (keeping the heat down).
Quite frankly, very few systems can take advantage of the 9800Pro's performance. You need a good solid 2.8Ghz P4 (AMD users need even more due to the lack of SSE2 instructions, or an AMD chip that supports SSE2) or faster system to really be able to push this card near its limits.
Nothing wrong with the 9800Pro or 9800XT. Fine products and definately the fastest you can get right now. The price/performance difference between the 9600XT and 9800Pro is pretty extreme. A $75 to $125 difference for a 15% to 20% gain in performance is prety expensive.
Of course, if you buy a video card once a year or less, then the 9800Pro or 9800XT makes more sense as you are going to live with that card for a long time. If you buy every 6 months, then it makes sense to buy the 9600XT as in 6 months, there will be a much faster card available.
Power supply. If you buy the 9600XT, power will not be so much of an issue. The 9600XT does not need an external power plug, which means it meets the AGP bus specification of 48W or less on power draw. This is significantly less than the 9600Pro/9800Pro cards.
The 9600Pro or 9800Pro are hungry. If you run a 2.4Ghz (or equivalent) CPU or faster, with a high speed FSB, I would recommend the 430W Antec TruPower as a minimum.
