That board is technically CrossfireX capable, but the PCIE 2.0 slot is limited to x4 speed. Essentially, the card in the 3.0 slot would have the maximum x16 PCIE-3.0 bandwidth allotted by the Z77 chipset, but the bandwidth of the card in the 2.0 slot would be lessened quite a bit in comparison. Such a setup wouldn't achieve ideal Crossfire performance, but there would be a gain in performance. Typically, you can expect a 7870 Crossfire configuration to scale upwards of 70- to 90-percent (sometimes nearing 100-percent) in many titles. A Crossfire setup using your motherboard would not yield the same levels of Crossfire scaling due to the slightly limited bandwidth of the 2nd card.
What you'd ideally want for a Crossfire or SLI configuration is a board that has two identical x16 slots, which would then split the x16 into a x8 + x8 configuration. MSI's Z77A-G45 and above boards have such a setup - they all use two PCIE-3.0 x16 slots. (So do many boards from other manufacturers.)
Lastly, you could potentially run into further bandwidth limitations for the 2nd card since it shares its 4-lanes (x4) with the remaining PCIE-x1 slots. Should you make use of additional cards in those slots, you would end up taking away some of the already slightly limited bandwidth.