The biggest increase will be in igp performance, which isn't relevant to most people, new chipset won't bring an awful lot with it, pci-e 3 maybe, to go with it on chip, to be absolutely not utilised by any gpu for a good couple years anyway. 7970 might support it, probably will, won't "need" it at all in any way.
There's going to be very little difference CPU wise between a 2600k and 3600k, but considering you could put the money towards a second 7970 instead, and have a gaming system 70-95% faster depending on the game, I know which way I'd go.
Sandy bridge is pretty awesome, the next "big" update to cpu performance will be Haswell, mid 2013, which should be bringing hex/octo cores to mainstream segment.
If you have a Sandy bridge, there's FAR better things to spend your money on for performance than an Ivy.
Personally, with the cash you seem to be able to save and what you have already, 2x 7950's would be the best upgrade path for you. I'm going on the fact you want a 7970 as you being a gamer. A 2600k is so FAR above what any games need right now, and for the next few years, that spending a load more on upgrading the CPU is where you'll see the least performance gain.