They are both Quad pipe designs i believe, but the X600 is dx9c based the 8500 is dx8 i believe.
The x600 has significantly more and faster ram, and probably faster core speed as well. But neither are 'fast' by modern standards.
Final comparision,
8500 - AGP, upto 128megs of ddr1, core 275mhz (quad pipe) ram 275mhz (128bit) dx 8.1
x600 - pci-e upto 256megs of ddr1, core 400mhz (quad pipe) ram 600 mhz (128bit) dx 9.0c
edit - yes i did just testing to see if you're awake there Zing
