The AV Quadras stand apart from the rest of the Quadra line with their AT&T Hobbit coprocessors and GeoPort serial ports. both the 660av and 840av can be overclocked.
These pages provide information on the maximum reliable speed for various Macs. This can vary from one machine to the next. Chipping is not authorized by Apple. Any modifications you perform on your Mac may damage it. Modifications are done at your own risk.
Be sure to visit The Clock Chipping Home Page, the definitive resource on chipping the Mac.
The Quadra 660av (identical to the Centris 660av) runs a 25 MHz CPU from a 12.5 MHz oscillator. With a 16-17.496 MHz oscillator, you can drive the CPU to 32-35 MHz.
The Quadra 840av has a 20 MHz oscillator which runs the processor at 40 MHz. The max oscillator you can replace it with is 24 MHz, to run the processor at 48 MHz, although you may run into problems with the serial ports at higher speeds.