The i5-3570K is a better processor, and is usually, nearly, the same price. The motherboard you've got is ideal for the 3570K, but not for the 2500K. If you are staying with the 2500K, change the mobo to the Z68 version.
If you are planning to overclock the CPU (whichever one), you will need an aftermarket cooler. The ones supplied, with CPU's are not adequate for overclocking.
Change RAM to 1600MHz, something like Gskill Ripjaws.