To overclock (may void your warranty!), add the following parameter to the cgminer command line:
Where xxx is the speed in mhz you'd like to run.  Valid values are 125 on up, and default is 550.

If you are running a RPI with our image, you can add this line to the "Extra cgminer parameters" section in the settings page of the web interface.

In a perfect world the hashrate is mhz X 768 (the total number of hashing units built into the ASIC).  Each of the 4 die has 96 cores, and each core has two hashing units, for a total of 768.  Note: It's normal to have a few defective cores per die, and it's also considered normal for some cores to produce occasional errors.  This means the maximum possible hash rate is 422.4 Gh/s when running at 550mhz, but in the real-world it will typically be lower due to the reasons I just mentioned.

Some boards will like certain clock rates and some will like others.  If you do experiment with values, and I'm not recommending you do, try small increments and watch the error rates and/or if the ASIC stalls.  When it attempts to draw too much power, the power supply can momentarily dip and cause it to stop sending work.  This is when you'll see the watchdog timer get invoked.

On an average board, the buck converters (take 12V down to core voltage) are usually the limiting factor for ASIC performance.  They put out over 400 amps of low voltage power for the ASIC's cores to operate with.  Right now in the present version of the firmware and cgminer, the regulators cannot be adjusted, but soon we will release firmware that supports adjustment of these regulators as well as independent clock speed adjustment per die.

We are also presently performing lab qualification of the silicon, so I'll soon have some numbers for you guys as to what the silicon is capable of.

The on chip programmable PLL is capable of pushing the clock rate to well over 1ghz.  (You would need about 2X the power and cooling to hit this rate though!)

-Phil