these are my lowest p-states tested and actually default in my rig:
I'm not shure if there is any advantage to manage two or more different profiles in k10stat (low and hig)
Your clock speeds in those pstates match the default clock speeds on the X3-720. I don't know if you changed any of the speeds while you were changing power levels.
Myself I changed clock speeds as well.
Stock pstates for X3-720 BE
A modified profile for saving power but the P1 and P3 states include a higher clock speed in the mix as well.
My thinking was that I should increase the clock speeds on lower power states to reduce the need to switch to higher power states. And since I could do it and still use lower voltages I figured that is a win-win.
I tested lower voltages and my rule of thumb was to pick pstates 2 voltage choices higher than the last one that gave me an error. I only did about 15 minutes of testing with each pstate but I found some of them would error out within seconds or minutes. I don't think I caught any errors between the 5 and 15 minute mark.
I also set up a few overclocking profiles in case I get into a very demanding game or transcode a lot of video and want the extra speed but even in those profiles I used lower voltages for most or all of the pstates. I'm not sure when I'll use them as the undervolting profile is doing me quite well at this point. I did have fun testing the limits though.
Oh and for anyone that notices that my NB voltage and CPU voltage match. I'm using an older motherboard that doesn't support split power planes so I don't get to run the NB at lower voltages. It costs me a few watts and that makes undervolting with K10stat that much more important.
I'd be curious to see the stock NB values for a X3 720 so I would have a better idea of how close I am to the idle power I'd get on a split power plane capable motherboard.