I have a k6-2 350 in my webserver (plug - http://www.beerpalace.org).
It's running at 460 (115*4) no problem. Running linux, and dnetc OGR (100% CPU usage).
I've had it up to 540 (120*4.5) at 3.2 volts, which is very very high, and hot.
As for CPU coolers, just about anything will work. You...