It's probably too old to handle the game. (Yes, i did see the part where they compared it to being an equivelant of a geforce 2, which is the bare minimum to play PS. I just think those bare minimums need to be updated a tad.)
PS uses OpenGL, which by searching the web shows that this particular card doesn't like it very well, but does say it can run it, although, again, it mostly doesn't like to. So that could also be the problem.
I'd suggest going into the PS Setup application, and turning down all the video settings to the lowest settings possible and see what happens. The game isn't goign to look very pretty, but it may be more stable.
As for getting a new video card, I don't even know if you could get an agp version of that. It seems nearly all newer cards are PCIe based. For that system, my suggestion would be to go with something along the lines of this:
http://www.newegg.com/Product/Product.asp?Item=N82E16814121525 . Make sure your mobo supports AGP 4x/8x though, else you may have to find something PCI based.
Edit, well, seems the 6600 does come in AGP. But I do agree, it seems a bit overkill for that old of a system at that price. The ATI I listed would do the same job and it's a bit cheeper. But thats all a personal preference really.
