The T-Mobile G2, brought by HTC as the Desire Z with HTC Sense, has 800 Mhz. A member of the XDA developers form wanted more, and started overlocking his handset. In the end, he got an amazing 1.42 GHz out of the processor.
With the attempt to overclock the Qualcomm MSM 7230 processor that powers the Desire Z/G2, you risk bricking your phone forever. Overheating is one of the dangers. If your phone is keeps working, however, its performance improves big time. Michael Huang, the man who is responsible for this succeeded overclocking attempt, runned a benchmark appliocation after he finished overlcocking. The results are inpressive. The handset gets a Linpack score of more than 225. At the moment, the Nexus One by Google is the best performing phone out of the box with 1250 points.
Source: Forum.xda-developers.com