What are the rest of the parts in your build?
all those cards really are overkill for those games, however if you really want to the 7870xt/le is the best, and at the moment on sale
Get the best graphics you can afford, because there will be another game you want to play in the future and you will be upset if your system cant do it as well as you want it to. I would say go for the 7870xt.
Another option, if you can wait, is for the gtx 700 series that is coming out in a couple months. That might be worth waiting for and give you better price options. AND if those are too high, the price on other nvidia cards will come down. Maybe pick up a gtx 660 ti.
Mother Board: ASUS z77 mITX Deluxe
CPU: Intel Core i5 3570k @ hopefully 4.3 GHz
CPU Cooler: Phantek TC12DX
PSU: Corsair 430 Watt
Hard Drive: 1TB Cavier Blue 7200 RPM
Case: Bitfeinex Prodegy Fire Red Steel
RAM: Corsair Vengaence Red 1866 GHz 8 GB
Sapphire 7870 XT since it runs cooler thsn powercolor myst edfition.