Well the main issue you're going to run into is Core 2s are locked, so that means you are going to have a tough time getting a lot out of OC-ing it. In addition, pre-built PCs like the one you are using are tough to OC as well because they tend to be steeped in features that limit OC capabilities. Unfortunately, the best fix for your situation is to upgrade your CPU. However, since you are using a Core 2 I'm assuming you'll have a motherboard with an outdated CPU slot and will require a motherboard upgrade as well. But still, with those in consideration, you can find a good mobo and CPU out there, intel or AMD, that will run you at most $150 together.
But if an upgrade is completely out of the question at the moment, I can't really...