That's an extremely attractive machine you have.
Unluckily the after that step up or ups available really require almost all to be replace.
If you now desire is slightly better graphics see what AGP cards you can find. The FireGL is a desktop card I feel (not designed for 3D gaming). In addition of a 2nd Xeon chip (assuming there is a spare socket) would be a good improvement, but I'm not sure how easy it is to find another 2.8 GHz Xeon 90nm chip?
Newer RAM/CPU would require a new motherboard. You do have DDR2 RAM (DDR2-400), but upgrading to DDR2-800 etc wonít help much with a 200 FSB.