That motherboard is useless if you don't want to overclock so you could get a cheaper one and a better GPU.
Not sure how much your RAM is but 2666MHz is overkill for gaming.
The R9 270 is the same GPU as the R9 270X, but the R9 270X is a higher clock speed, so you can OC about the same amount.
Remeber to keep an eye an temps.
Z97-G55 has 3 PCI Express 3.0 x16 slot while the Z97 Gaming 3 only has 1 and 1 PCI Express 2.0 slot.
That only matters if you plan on SLI or crossfire.