For motherboard I have no clue...Im searching for a USB 3.0 Sata III(6gb) and that has only 1 PCI EXPRESS 2.0 slot(it can have 2...but the price has to me similar), I'm the type of guy that likes to have only 1 powerfull videocard...these are the options I got at the moment:
If you want to use the 2500K, you NEED to get a P67 chipset motherboard. The best/cheapest option is the one that JackNaylor included, the P8P67 Pro. H67 motherboards CANNOT overclock the 2500K, and only run RAM at 1333 speed. If you can't stomach the budget there, you're going to have to look into an AMD build or maybe even an i5-750 build.
the difference between the sets is that the more expensive mushkin runs at Cas 7 which basically means it's more efficient buy the cheaper one the memory latency doesn't matter very much for overclocking SB moderately
Don't bother buying a heatsink the stock will do fine 4 ghz max should be easily attainable, not sure what heatsink you have but you can just buy a 1156 conversion plate (1155 and 1156 mount the same way)