The fastest CPU for that mobo is 2.0 GHz, if you have BIOS 14 or later.
The board can NOT be overclocked.
It uses the (expensive) RDRAM.
The speed difference is not worth it.
Keep that system as a (reliable) backup and build yourself a nice new system with an AMD64 socket-939 processor. You can go all the way and get a board with PCI-express (or SLI) graphics and DDR2 memory.
A bit cheaper is going for a board with AGPx8 graphics and normal DDR memory.
The likes of Abit, Asus, Gigabyte and MSI have good boards here.
You will need a fairly powerful, good quality PSU for this.
Plenty of posts on this subject in this forum.