The last Brisbane I had (Athlon x2 5000+) got to 3.3Ghz with an H60. Voltage was around 1.55ish, and heat wasn't really an issue. I think the board couldn't hang after 3.3Ghz.
As for finding the reference clock, look in your BIOS. You should see something similar to the pic below:
Some BIOS options are listed differently on certain boards / manufactures. What your looking for would be what says "243" in the picture. This is called the Base Clock, Reference Clock, CPU Frequency etc. That number multiplied by your "CPU Clock Ratio" determines your CPU Clock speed.
Example: 200 x 13.5 = 2700Mhz or 2.7Ghz
208 x 13.5 = 2808Mhz or 2.8Ghz
Keep in mind, any increase to the default "200Mhz" reference clock increases your Hyper Transport bus, NB Frequency, PCI-E frequency as well as your Ram frequency.
