What's the fastest rig you can buy? And has anybody tried overclocking, etc.?

I've yet to buy all the parts and get this up and running. But I'm thinking about longevity here. There is a good chance that the released version of the os will change and no longer boot on current hardware. So I'd like to set up the fastest rig possible, and then overclock it and perhaps watercool it.

Anybody else with the same goals and or success?