I'll toss in my recommendation for the Haswell i5-4670k.
However, if you can find a good deal for the Ivy Bridge i5-3570k, then that will work too. The average performance increase to Haswell is about 6%. It can also reach higher overclock speed. It appears the percentage of i5-3570k that can be overclocked to 4.5GHz is higher than for the i5-4670k based on user feedback.
Going with Ivy Bridge (socket 1155) will mean you cannot upgrade to Broadwell in late 2014 (socket 1150). However, the improvement in CPU performance vs Haswell will likely be fairly small again (~6%) since the goal is to reduce power consumption. The integrated graphics core will probably increase in performance by about 30% - 40%, but it can't compete against a moderately priced graphics card. Therefore, even if you buy the i5-4670k, you are not likely to upgrade to Broadwell anyway unless the i5-4670k somehow dies.