Welcome to the forum, WetbackMountain.
As stated above, Intel and AMD use completely different slots for the their processors; one won't fit the other. So, the first thing that you will require is a new mobo w/slot 939. I would suggest DFI, even if you don't OC. They have a killer BIOS. Then, comes the processor.
Here it depends on your budget. If you go with an AMD 64, you will get better performance with less GHz's than what you have now. The reason is, AMD 64's have the memory controler on the die. Therefore, the memory controler runs at the speed of the processor.
You have two choices: a) single core or b) dual core. I recently purchased the X2 4800+. It is 2.4GHz. The dual core is great if you muli-task. However, they are more expensive. The dual core will future proof your computer, because the technology is just now getting in full-swing. Only a few games take advantage of both cores at present, but developers are beginning to crank them out.
Go to this link for some good info on processors: