For all practical purposes, the difference between a PCIe 2.0 and 3.0 slot will have little bearing on the performance as seen by the user. Installing a HD7870 into the PCIe 2.0 slot, for example, will have minimal measurable impact.
Essentially, this really isn't an issue.
Alright good now the only other problem i see would http://www.newegg.com/Product/Product.aspx?Item=N82E16814125418
be able to fit into the case. the case measurements are 7-1/8"W x 13-7/8"H x 16-7/8"D.