I did the same, inputting the folling entry into the CPU Power database txt file, saving, then reloading it:
AMD Sempron 2800+ (1.6); 1.6; 62; 2000
After selecting "AMD Sempron 2800+", I entered 1V & 2000 Hz -- and it calculated the power to be 24.2W. Not 29W.
Note -- the max power given by AMD is the same 62W for a bunch of Semprons, so you can assume it's lower for the 2800+ compared to the 3100+. How much lower, just guess.
I'm using the AMD Palmero core, 90nm, 1.4 volts is the default voltage, and its actual operating speed is 1600. Yes it probably is lower as the 3100+ i think is 2.0 or 1.8 actual ghz.
i entered different values but it makes sense that 3100 is 62 watts.