What are you using this system for? Whats your budget? Do you have a PCI-E slot or AGP? What power supply do you have?
The best upgrade you could do would be to get a modern system, even a low end system today would blow a high end P4 away.
It's a 16x PCI express and has a decent power supply. Mainly used for internet, movies, adobe photoshop. Nephews when over like to play games. Budgent not an issue. Wanted to upgrage new system probably next year. This PC still fine, worth a few bucks to keep operating.