As Dr says, the Q6600 runs very cool anyway. I am using the stock cooler and I am running at 3.0GHz (stock is 2.4GHz) 24/7 with no problems. I have left all other buses running at their proper speeds, just clocked the CPU FSB to 333.
I am running Fedora 8 x86_64 and it's working marvelously well. I have all the fancy compiz-fusion wobbly firey desktop effects in use, and I have a virtualised XP Pro running that I RDP to on another side of the cube for running Outlook and a couple of other apps.
Not sure what you're into, but I'm finding all sorts of marvelous stuff for GNU/Linux right now. Ardour, Rosegarden & Tuxguitar for music stuff. All sorts of video/music stuff.
Failing that, go for XP. Stay well clear of Vista. All Vista is doing it putting a lot of effort into stopping you doing what you might want to do, e.g. protecting/encrypting media streams through the machine.
Also Vista has pretty much the same memory limitations as XP, since this is dictated by the architecture and not the O/S itself.