I would say that a PCI-e 1.x slot would bottleneck a HD 7970. However, there should not be any issues with bottlenecking if you stick the HD 7970 in a PCI-e 2.0 slot.; there is enough bandwidth for it.
Overall, the increase in processing power from Sandy Bridge to Ivy Bridge will probably be around 6% in my opinion; some people estimate as high as 10%. Most of Intel's efforts went to the Intel HD 4000 graphics core which is estimated to be about 60% better than the Intel HD 3000; at least's that's Anandtech's opinion.
The smaller die size could be mean that IB CPUs may be clocked slightly more than SB CPUs. Perhaps there may be better overclocks as well, but that is pure speculation at this point in time.
If you can wait for IB, then wait. You'll get a little better processing performance. Otherwise, just build your PC around SB. The next big step up in CPU performance will not happen until Intel releases Haswell (mid 2013).