Ok, you could get just a GPU, but it would have to be small. Pre-built computers have bottom of the barrel PSU's and any average or high end GPU will quite literally make the PSU blow up, taking down half your computer from the surge.
If you want to upgrade your PC, you would need a new motherboard to start. Now, you got very lucky that the mother board right now is ATX sized, so you won't need a new case unless you have issues fitting a gpu inside.
Your current RAM is DDR, which is so past generation that I don't think you can even buy it. You will need DDR3 Memory.
Now you need a CPU. Forget the APU, they are for office stuff, not gaming. The APU is made for Microsoft word and Internet Explorer, just so you get an image on your screen.
Then you can get away with a $50 GPU.
The hardrive uses a SATA connection (very lucky once again) so you can re use that. I am pretty sure the CD ROM is SATA but not sure. If not, it is a $20 piece.
Now with all this new stuff, that crappy PSU ain't gonna cut it. You will need that.
This is why I went from thinking about upgrading my old PC and decided to build my own. If you can't get the power you want in a $50 GPU, you pretty much need a new computer. If you want to build a computer, give me your budget and I'll try my best. If you just want a GPU, I can do that too.
Edited by The Bacon Man, 30 June 2013 - 01:09 PM.