These cards start with a 16.6MHz crystal, and with a 3x multiplier that gives a 50MHz bus. By replacing the crystal with a faster one, the bus can be increased to around 55MHz. I have a report from one user who put in an 18MHz crystal to give a 54MHz bus, and everything was fine. With an 18.43MHz crystal which gives a 55.3MHz bus he had some problems.
Then you can change the bus:processor ratio by following the info on Takashi Imai's web site on the Inline Cache Processor Card Modification. The board comes with a 5x multiplier set to give a base 250MHz. With the 54MHz bus that comes to 270MHz. The clock chipper I heard from was able to change the ratio to 6x (324MHz), and even as far as 7x (378MHz) without any problems.