temprerature gagues are not 100% accurate, some are prolly not even 50% accurate, like when Windows says 'x time left' but it ends up being completely different
Probably more to do with your board, ram, things such as that besides your heatsink....the 2600+ is stock 2.0 right? so 300MHz gain? Unless you're saying that your temps were too high to try it in the first place and that heatsink allowed the temps to come down far enough for you to OC