Your gonna be limited by your CPU for a lot of newer games, especially if you want to play at higher detail. The bad part is you can't overclock the CPU either. I wouldn't buy anything over a 5770/460 if your going to use it with the processor.
As far as a psu goes, it looks standard size, but I'd do some googling around to see if anyone has commented on swapping out the PSUs. Generally the pre-fab systems aren't very mod friendly.