you dont necessarily need a water cooling kit, it depends on how much you want to overclock. If you are thinking of doing a light overclock of lets say anywhere up to 1.2 ghz above the stock speed then you will only need an aftermarket fan/heatsink. some air cooled heatsinks can actually go up to 2 ghz above stock speeds if you get the right one and the right thermal paste. Now if you want to get a water cooling kit then by all means more power to you. Corsair has the hydro series water cooling kits: h40, h50, h60, h70, h80 and h100.
the h 40 to the h70 are "basic" kits for starting out and i would recommend one of those for you while the h80 and h100 are larger, especially the h100, and more expensive, at least 100 dollars