Hello!
For MAXIMUM performance, either a i7 920xm or i7 940xm would be the most powerful upgrade. Though this is the most powerful upgrade you can make, I would not suggest buying it as the cheapest 940xm I could find was $144 and the cheapest 920xm I could find was $83.
If it's a cheap upgrade you are looking for though, I'd suggest if you're not already running an SSD, that you install one as it'll perform WORLD's better than an HDD and reveal a lot more performance.
If you already have that upgrade though (or just need better CPU performance), and are looking for the best bang for your buck, than I'd suggest the 820qm or the 840qm. I found the 820qm for as low as $38, and the 840qm for as low as $50. Which ever one is in your price range should be a decent buy.
Before buying any of these chips, I'd suggest quickly checking to see if your cpu isn't soldered to the board (just as a precaution though it is likely that it uses a removable socket).
(NOTE: All the prices represented here were based on deals found on ebay)
Hope this was of some help!
- Nate of Ralleon Custom Built