Personally, that strange shape of the power supply limits you a lot. If I were you, I'd get a new case (gaming case for good cooling), something like this:http://www.newegg.com/Product/Product.aspx?Item=N82E16811129181
Then you should get a decent power supply from a good brand (those include Antec, Corsair, XFX, OCZ, Seasonic):http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027
- $49.99 after rebates.
After that, it all depends on how much you want to spend for a graphics card and which games you're gotta play.
Your options main options:
HD6770 - $115
HD6870 - $150
GTX 560 Ti - $200
And all the rest above that budget.