Just the GPU should be fine for now. It's a quick, easy upgrade with immediate benefits for gaming. I just upgraded to a GTX 980 and still have the same i5-3570K as you (Ivy Bridge; Sandy Bridge was the previous generation). Skyrim runs great at 1920x1080 with S.T.E.P., and I've got the quality version of the Vividian ENB on top. Recommendations for the i7 are popping up for some games, but I wouldn't worry about it for games that aren't out yet. If you see reviews for a game you want that's confirmed (by gamers, not just official recommended specs) to be crippled by an older CPU, then you can consider upgrading your CPU/mobo.
