AMD FX is a horrible choice - there's way too many issues associated with it. A combo idea is OK but you don't get total control over the hardware you want - especially since a lot of the time they generally include things like crap PSUs, RAM with tall heat sinks, and things of that nature.
Would 650W be enough power too btw? Because I found a combo deal for a 650w with the same case.
650W is plenty of power for a single 7870 - if you want dual - an 800 would be better but you have to make sure that the PSU you choose is certified and rated 80+ bronze or better, a PSU sans rating will be disastrous.