In G5 models BUS-speed is bound to CPUSpeed (ratio), which is coded into CPU-module.
On a PowerMacG5 1,8GHz you have Bus-speed of half (900MHz). Changeing the CPU-Modules to a 2,3GHz-version, rises up BUS-Speed to 1150MHz automaticaly (Northbridge can do that - working on such a G5 at moment).

So its more a issue of the CPU than of Northbridge.