If you are looking for a cheap upgrade, you can choose the memory. If you are using DDR2, make sure that memory has a low latency. I don't recommend DDR3 because is still new and and still have very high latency. Wait for the next generation's of DDR3, unless you need it for some reason.
On the other hand, if you plan to make a more serious upgrate, you can choose for a processor. In this case, for games(like Crysis) and normal applications, I recommend a Core 2 Duo E8x00 series (2.4 GHz, 3.0 GHz, etc.). Or Core 2 Quad Q9XX0 series for games and other applications that generally require a lot of processes. I also recommend to upgrade the memory if you choose a processor.