Are you unhappy with the picture on your current setup? I use a 6600GT because I need it to view HD content with VMR9 and my 9700pro was not up to the task. DVDs were a little jumpy, i.e. pan shots had "microstutters". The picture quality was fine except for the stutters, with HD content, it was unwatchable. All that has gone away with the 6600GT.
If your DVDs playback smoothlya nd the picture is good, I wouldn't bother with the 6600GT. You might keep an eye out for a Northwood CPU though, I'm pretty sure the 1.7Ghz P4 is a Willamette which is probably not much better than a Pentium 3 1Ghz...Seriously. If you could find a used P4 2.0- 3.06 Ghz Northie, you would be much better off.
You also need a bit more RAM. If you could find another 256 MB (2 x 128Mb sticks) used, you'd be better off too. New Rambus is an arm and a leg.
Recently I upgraded my gaming pc to an Asrock ULi chipset baord which supports AGP and PCIe (both natively, so it's not some ad hoc design). The board was 70 bucks and I picked up a A64 3200+ for 152 bucks. To upgrade your CPU and RAm you are probably looking at at least $150 (even for used) and probably upwards of $200. For approximatley $270 you can get a that Asrock mobo, an athlon 64 and 1Gb of DDR400. You could reuse your 9800pro for a while while you saved up enough money for a PCIe 6600GT.
Regarding your mobo: I *THINK* a 6600GT would run in there. It is AGP 2.0 spec, but I know some of the newest AGP cards will only run at 0.8v (i.e. AGP 8x Only)and I think that mobo will be a 1.5v AGP slot. Any video card listed at 4x/8x support will work. Any videocard listed at 8x only will not.
