You can easily overclock a 2500k to about 3.7 GHz on the stock heatsink (although don't expect it to be quiet).
Hmm those are some tall VRM sinks... I'd measure first but look at the Cooler Master Hyper 212 Plus, Corsair A50, Corsair A70, and Prolimatech Megahalems (+ buy a 120mm fan) - the heat pipes bring the fins up pretty high over the socket so they should fit over those heat sinks.