Well depends what that computer is going to be used for.
So if it's just for internet browsing etc then a simple GeForce fx5200 will be fine. If u play games or do other stuff that requires a fairly good vid card then I'd suggest a 6600GT or if u have the money to spend then something from the 6800 series or the 7800GTX.
And as for the PSU some companies make it so that it's not easy to change. But a little work with a hammer and u should get it out of there.