CPU+mb

Your 7950 (or any other current GPU) cannot even fully utilize PCI-e 2.0. The only benefit to 3.0 has been with very high end CFX and SLI configs, how ever the benefit is extremely marginal and nowhere near worth the added cost. For the time being it is my opinion that PCI-e 3.0 is nothing more than a marketing gimmick on high end boards.

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But... to get what you want you will need a pcie3 capable motherboard as well and an Ivy Bridge Intel cpu ... and thats going to be tough given your budget.

...but in answer to your question, a z77 board will do the trick.

One of those plus an i5 2500k or i5 3750k is possible within your budget. Together with your 7950, you will have an excellent gaming rig.

Nevertheless, the other posters are right. 7950 cannot even max out pcie 2 bandwidth, so you will get no performance benefit over a z68. Therefore, a z68 is a perfectly good alternative.