It depends on what speed the pci-e 2.0 port is running at. If its running at x4 speeds, then dont bother.
If its running at x8 you can probably do it with likely no performance loss.
If its running at x16 then it will work at full speed no question about it.
pci-e is pci-e, the revision of it doesnt matter. Even if pci-e 3.0 has double the bandwidth of 2.0, if you only use 15% of 3.0 (30% of 2.0) its not going to make a difference, since both use the same bandwidth and have the ability to use the full amount of bandwidth.
The 7870 is not that high end of a card, it really probably only uses like 30% or so of pci-e 2.0s bandwidth. Even the gtx titan doesnt use the full 2.0 bandwidth (by a decent amount actually).
pci-e 3.0 wont be useful for probably another 2-3 years.
most all boards ive seen with pci-e 3.0, when running in cf/sli onyl run at x8/x8 (which is equal to pci-e 2.0 at x16/x16 and i see that all the time, atleast on amd boards)