Difference between sandy bridge, ivy bridge, and haswell performance wise will be very minor excluding the iGPU performance, and power consumption.
Wait for haswell to come out it, which is soon I believe, and it will last you 4 years easily.
If you need a processor now, get a 3570k(this will last you 4 years easily too). An i5 is better for gaming. If your doing encoding or stuff that will utilize hyper-threading get an i7.