To my knowledge, processor power is based totally on who makes it, architechure, and a few
other things, no matter how chilly you make it, it won't techncally get more powerful, but the
clock speed will go up a bunch.
www.futuremark.com is a decent site to check out
people's benchmarks, see what systems the use, compare them to others with the same
system but who overclock, etc.
Pentiums overclock like mad, but don't show a huge increase, while Athlons overclock
minimally, but show some decent speed changes. Windows is on your HD, no, you wouldn't
need to re-install it :P. My advice, is just to get a high-end processor, like an Athlon 64,
then get a Motherboard that will support the Athlon 64, get a very nice heatsink/fan
combo for it, AND LEAVE IT ALONE. Don't bother overclocking your processor because
you'll probably end up ruining it. With it very cool, you'll be able to run thousands
of intensive programs and not have the temp go up much. Case fans are good for moving
warm air out of the case, but have little affect on keeping component pieces cool.
Hope this answers something.