It would really depend on how patient you are and just how much work you're doing with the CPU. If you're a gamer, you're not going to notice a huge change in performance between the current-gen Core i5 and the next-gen Core i5.
Yes, the Haswell-based processors will definitely be better, but that depends on how they're being used. CPU intensive applications will definitely show a performance increase, but it also comes down to price/performance ratios.
A Haswell upgrade might cost hundreds of dollars more than the current Z77 motherboard & Core i5 3570K processor would today. You'll have to account for that when making your decision. I would suggest getting the current hardware, and letting Haswell unveil itself and show just how much more performance is included.
The money saved could go toward an SSD if you don't have one already which would make a huge difference in overall performance.