Motherboard = Asrock 775Dual-880 Pro
Pentium 4 3.00 GHz
2.99GHz 2GB of RAM (DDR 2 PC2 4200)
Current Graphics Card = Matrox Graphics Millenium II PCI
Power Supply = Model LC-8460BTX, 450W
Windows XP Professional
Direct X 9.C
What do you mean, that is the best your mobo can do? You have a PCIE slot on it. The more likely limit is your PSU; I'm not sure it would safely handle much more than an 8600GT. The 9500GT or HD4670 don't need an auxilliary power connector either, so one of those would be better.
Manufacturer...ASRock? Their link is now broken. In any case, the limiting factor would be power. A 9500GT would perform similarly to an 8600GTS, and an HD4670 would be a LOT better. Both of those use less power than an 8600GTS.
I would say its a toss up. on crysis there was about 1/2 a frame per second difference. on call of duty 4 it was almost twice as fast. on Devil May Cry 4 it was the opposite the 4670 was faster. on 3dmark06 it shows a 3k difference. on vantage about a 500point difference. also the 4670 is dx 10.1 vs dx 10 and it uses a little less power than the 9600gt. heres the review. http://www.legitreviews.com/article/792/1/
Of course the rig it was tested on is much more powerful than yours.(even mine for that matter)
Obviously you wont max out crysis on high or ultra high settings, but on medium it should do decent enough.
either way its your choice on which one you want. hopefully this helps you decide. Good luck.