A 9800Pro is ~roughly~ equal to a 5900U; 5900NU is ~roughly~ equal to a 9800 Non Pro. ATI's highest last gen card was the 9800XT, which itself was ~roughly~ equal to a 5950.
I upgraded from a 5900SE AGP on a 2.4b P4 to a 6600GT PCIe on a 3.0 Prescott, so I couldn't compare platform to platform. Honestly, I did not get a "great" performance increase going to the newer platform, but I was able to max out image quality at 1280 x 1024. With the 5900SE I sometimes had to back down on IQ in some games (Far Cry comes to mind). You will get an increase, but you probably won't see a $150 performance boost. I had to upgrade due to PCIe.
If you run older games (DX8 or lower), the 5900 will go head to head with or slightly outpace a 6600GT in some instances at low to medium resolutions. I had a link to site that compared a 6600GT vs 9800Pro vs 5900XT/SE running with a 2.4 P4 which I found VERY informative before I bought my 6600GT. If I find the link I'll post it here.
In either case, 6600GT will punch out any 5900/5950 running PS 2.0 or higher. Unless you've got extra $$$ to burn I would wait awhile until prices drop more or newer GPUs bring bigger bang for the buck.