Depends on how far you want to go, the best value is the cooler master hyper 212 plus or evo, which are 30/35 and will get the 2500k to 4.5 so the 2600k in that will prob go 4.3-4.4 i guess stable. Then you get into the high end air and closed look water coolers, which cost 60/70 to 120, and should get it to 4.9-5ghz, then you get to the home made systems where you buy the individual pump, tubing, res., block, radiator and a ton of other bits, which costs out the arse but will get you to 5+ghz rock solid at great temps, and allows you to loop in the graphics cards too so you can overclock them too and get them cool too.
EDIT: You will also need some thermal paste which you can get for a few bucks, or some good stuff for like 12 bucks, or the great stuff 30 or so.
Also you might be interested in at least adding 100 to get a 60-64gb solid state drive, which is basically a virtual hard drive that uses memory chips like a usb drive, they increase load times on everything on them like crap, boot will take like 30 or less secs i think.