What can I upgrade this GPU to?

If you can invest around $60 to $80 try out these:
ge force gt 520, gt 430 , gt 440
If an AMD fan then , HD 5670 is a great buy.

If you want to play games, I'd strongly suggest you look at another machine, the one you have linked to is far too small to withstand any useful, games related upgrading.

Dang... this computer was pretty good besides the GPU

It's not a bad little setup, but the case is just too small to accept a decent gaming card or an upgraded PSU to power it...Sorry, looks like you're still shopping.

Not a good idea.

220w psu won't power anything extra.
The case is <4" wide and would need a low profile graphics card.
Amd does not make good gaming cpu's.

It is ok for a basic pc, but for a gaming pc, build it yourself.

What kind of games could I pull off with this system?