The Power Center 180's have a 60MHz oscillator that gives a 180MHz machine through a 3x multiplier with the bus us running at 60MHz.
Following the 604e Card Modsby George Currey, Jr. on the PowerPC.org page, I've heard of 180MHz cards being clock chipped to 240MHz. Without changing the bus:processor ratio the bus is pretty much maxed out, and it is unlikely you will get past 190MHz.