I, also had the stock ATI 9000 for years in my old MDD.
I opted for the ATI 9600 Mac Pro after reading many bad stories about the 9800, with it's fans stopping, and the running-hot issues reported. At the same time, I am sure there are still many users here, who still use the 9800 without any issues. They are both much better cards than the 9000. Much better OpenGL support too. (better graphics capabilities) for certain sim-games, CAD, 3D stuff in general. And, no fan needed, although I use a PCI case fan, just in case. I run 2 monitors just fine with it also. Core Image and Quartz Extreme-supported.
The only other card I may consider now is the geforce 7800 GS, as mentioned by "Nadav" but, It may be a bit pricey, and hard to find .
Just my 2 cents, best of luck with it.