on your microprocessor, memory and graphics card. What's your framerate? When I switched from Athlon 64 3700+ to Opeteron 175 I got a 10fps increase in framerate. But I was running an nVidia 7600GT graphics card. So:
Priority 1: memory to 2 gb
Priority 2: graphics card or processor equally. Choose which one comes first by price.
Also, what slot type is your motherboard. I don't think you want to upgrade the processsor on an orphan motherboard most of the time. What kind of graphics board slot do you have? PCI-e means time to spend money as your card will be upwardly mobile. PCI or AGP means you should be scrutinizing motherboards.
One good thing about Acer is that they use standard components. You won't get stuck buying a new case because a replacement motherboard won't fit.
The result is a computer you can afford to keep up to date a component or two at a time.