Bladefish,
You could always go for the cheap upgrade: Find out the highest cpu chip your motherboard will support (that you can afford). If it offers enough of a jump in horsepower from what you currently have, then all you need to do buy the chip, flash to the latest BIOS, and swap cpu's.
Of course, you don't have to get the absolute highest rated cpu -- most important is finding the "best bang for your buck".
The beauty of this plan is that you can buy the cpu on Ebay, then sell your old one and recoup some of the cost.
No matter what you decide --
Good luck,
JD