Well depending on what you want to do, I would upgrade the CPU, Video Card, RAM, Motherboard, Case, Hard Drive, Fans, Power Supply, CD Drive, and sound card...
Nah im just playing... but seriously, I would look into upgrading your video card, and possible you're CPU.
I'd keep the cpu and possibly get into overclocking, maybe a new cooler for it. $300 to a cpu and gpu doesn't live much an upgrade and either area imo.