IF you want to stick with Intel, you wont have any sensible upgrade unless youd o a lot of multi tasking then a dual core might be nice but you'll have to change mobo/cpu/ram.
Goin AMD you could keep the ram but I dont think the cost associated is realyl worth it. SInce your system is rathe good...
If you're a gamer a video card will be the og greatest help!
Can you explain what this computer is used for and whats motivating your need to upgrade?
Asus P4P800DX, P4C firstname.lastname@example.org, 2X512 OCZ PC4000 3-4-4-8, MSI 6800Ultra stock, 2X30gig Raid0