I can see your point and understand your strategy but by opting for an 'A' series chip now you'll be stuck with a slower Quad cored CPU with, currently, no real upgrade options short of a better CPU/MB later.
I really, really, strongly advise you to go for the FX option for these other reasons:
Right now most systems will get along just fine with 4Gb of memory-it IS a little restrictive, but by dropping down to 4Gb from 8 you can move over $70 of precious budget to where it can do more immediate good.
Quite a few here look for a top flight motherboard when what they really need is a mainstream or even entry level one, make a list of things YOU want for YOUR system then see which 'boards match those requirements, you may be surprised how little you actually need.
There is plenty of inexpensive and good cases out there, sorry to sound harsh but putting a budget build into a expensive, top class case makes no real sense: The money would be better used elsewhere to enhance performance.
As always, check the prices, the graphics market has always been volatile with prices changing on a daily basis and there is no shortage of special deals out there, so keep your eyes peeled for those bargains.
The R9 270 is, essentially, a rebranded HD7870, and the R9 270X a rebranded HD7870 GHz Edition, the performance is practically identical.
Another pace you can trim the budget is the mouse/keyboard; of all the parts in the build they're the easiest to replace and potentially the least expensive.