If you've got the motherboard already, then you should be able to flash it to handle i5-3570 which would be a slightly faster chip.
With the right motherboard and CPU cooler the 2500k is going to be the best bang for the buck. Easily reach 4.5Ghz or so.
i7-2600k would be similar, and it might be the better long term investment with the hyperthreading. Newer games will be able to take advantage of that.