Very powerful system there Gaming_Freak
Though I don't know much about the power supply to comment. That said though, I tried to find something to skimp on to get the price down, but there isn't. That system sounds top notch.
Though if that other PSU is better but cheaper, go for it.
You definietly couldn't do better than the 7800GTX right now, though do note in the next month or so, another ones going to be out. but just don't be worried by that just yet
You can of course because it is SLI compatable, get another of the same card and boost games even more.