VRAM is a resource your GPU uses and is one that as long as you have enough of it, more doesn't impact performance. However if you run out of it, it's a royal pain in the bottom.
The reason I like more VRAM when using high end cards such as the ones we're looking at here is if you get a second GPU, you don't get anymore VRAM than you had on one card. Example, 2* GTX 670's with 2gb VRAM each in one system has only 2gb of VRAM usable, not 4. So if you were to get a second GPU in the future, presumably the performance of 1 card is not enough (hence the second). However in this future games will require more VRAM and to run max settings more VRAM gives more longevity to your GPU if you just add more cards.
Both are exceptional cards. The 670 is better, the 7970 will last you longer if you end up running multi gpu setups or if you game in 3d.
It isn't the 7970 though lol. I'd say get a 3570k though. Save yourself a few quid. Hyperthreading isn't' used for gaming and 0.1ghz extra clock on the 3770k isn't worth near £100 extra. Also with that £100 processor saving, you could have 2* GTX 670's which is a faster card in general (albeit with less vram, but 2gb will still last a long time). Hell you could even wait a few days to see what the 660ti GPU ends up looking and priced like.