7870 Crossfire will be overkill and bottlenecked by the CPU. As for single, there is a good chance that the CPU will not bottleneck at stock. But if you do though, overclock the CPU, you can't go wrong.
PCI-E 3.0 cards shows very small difference when used on a proper 3.0 slot compared to the 2.0, so you shouldn't worry much.
I have already overclocked CPU - it is running at 3.95 now. So, if CF is overkill should I go with 7950 or 7870 is still ok?