Getting another hard drive is probably what I would do. My second vote is to upgrade the processor to a Phenom II, especially if you have a socket AM3 motherboard. Upgrading to a Core i7 would require an OS reinstall, new motherboard, and possibly different RAM (but I would suppose you already knew that). If I had a 3rd vote, it would be RAM. 4GB is plenty, but there are small benefits from having more with your configuration (specifically because you have a 64-bit OS). Else for vote number 3 I would go with fan upgrades, and possibly a PSU upgrade.