If you like overclocking, and you have some spare liquid nitrogen just sitting around, why not just overclock a Core 2 Extreme to 5GHz?
That's exactly what Universal ABIT did at this year's Computex. They took a Core 2 Extreme X6800 (2.93 GHz) and strapped it to one of their AB9 QuadGT motherboards and throttled it all the way up to 5 GHz while cooling it with liquid nitrogen.
They were able to keep it running continuous Pi calculations for some time without any issues. This isn't exactly the greatest achievement in the history of overclocking, but it's still pretty cool.
CPU Cooled with Liquid Nitrogen; 5 GHz Overclock [via ministryoftech]