Wrong, that's a socket 939 motherboard. That's pretty much the fastest CPU for that socket. That CPU regularly goes for over $100 on ebay. You're going to have to come up with something else.
Personally I'd pull the motherboard out, sell that CPU on ebay, buy a single core CPU for it for like $15. Repurpose that mobo and RAM into another box. Then buy some other cheap AMD combo.
Edit: I looked closer at it, If you want to repurpose this the way I suggested you'll have to use it in this case. Apparently this is somehow a microBTX case and motherboard, so you'll have to build something else to begin with...
Thanks for your help guys. I appreciate it. Can you suggest a motherboard that would fit this case that would allow me to run a either an Intel E8400 Core 2 Duo or an AMD Athlon X2 5600+, all the while retaining the same RAM?
No, that's what I was saying. It's a microBTX case and motherboard which are dead standards. You'll have to get a new case and power supply that are ATX instead. Also being a socket 939 it's using DDR1 so you couldn't carry that over to anything newer.
As far as just trying to upgrade it you're in the worst position possible because it's pretty much maxed out besides adding a graphics card. Though if you were going to use it that way I'd only add a HD 4670. I'd still do what I suggested before though personally.