I have an X3 720 with an unlocked 4th core, so it's pretty similar to your CPU. I am currently @ 3.5 GHz with 1.424 V. I have a Core Contact Freezer and my max load temp is 43 C.
For Phenom II Quad Cores, you should stay below 71-72 C.
As for how far you can take the CPU, it depends on the chip. In brief, it's all trial and error so try to raise your voltage and your clock, test with prime 95, then see if you can undervolt further.