It depends on your motherboard. It's a slot 1 so you can get a 1 GHz cpu if your motherboard will support. You probably will need to update the bios to get a 1 GHz cpu to work. I looked at your specs and it says the cpu is upgradable but it doesn't say to what.
A 1 GHz slot 1 cpu with heatsink and fan cost $397. You can get a P4 2.6 for about $175. Next week the price of a P4 2.8 is supposed to drop to P4 2.6 prices. I think you would be better off getting a new pc.