So IBM and Georgia Tech have clocked a processor at 500 Ghz running in sub 451F temperatures. The same processor runs at about 350 Ghz in room temperatures. And, amazingly, this processor is silicone based. Meaning it's not going to cost more for materials than the average processor does, once production lines taking advantage of the technology are set up.
BBC Source
Georgia Tech article with technical information
Don't get too excited though, since this isn't a PC processor. While the technology was originally designed for use in PC's, it was deemed too power consuming for practical use. Currently these processors are used primarily in communication. Though it's possible that with other technical advancement, and proper application, we could see 500 Ghz processors in PC's in about 10-15 years.
