I've finished assembling and stress-testing the new home server yesterday. The i3-2100 CPU runs at 36 deg. C idle and up to 51 deg. at full load. I then tried to undervolt it, but I think I'm doing it wrong...

The BIOS claims the default VCore is 1.15, CPU-Z has it at 1.05.

I tried setting it in the BIOS to 1.05. A reboot later and the BIOS confirms it's been set to 1.05, but claims it's at 1.07. CPU-Z has it at 1.05. No change in the temps.

Lowered it to 1.03. The BIOS now claims it's 1.04. CPU-Z says 1.05. Temps went up a bit - 37 at idle, 52 at full load, but it's possible its due to a slightly higher room temp.

I've seen claims that the Sandy Bridge family and especially the i3-2100 can be undervolted to reduce the power consumption (which I can't measure) and temperature, but I've yet to see a guide. While I've uoverclocked some CPUs in the past, I've never played with the voltages, so either it can't be done or I'm doing it wrong. So, any idea as to what could be going wrong?

The motherboard is Gigabyte H67M-UD2H-B3, the BIOS is ver F4 (the latest).

There's not a whole lot of difference between 1.03 and 1.15 (and that's assuming there's no automatic undervolting by default). Some CPUs/boards allow you to drop the voltage lower.
You should still see a difference if you're persistent enough in your measurements (you'll need a lot more than two if you can't control ambient temp) but it shouldn't be large.

Are you going to run a distributed computing node or something? If not, you won't save much electricity by undervolting even if you do it right. Your temps are fine and you aren't on batteries, solar power or something so why do you care?

Undervolting may jeopardize the stability of your system by the way. If you don't know what you're doing and the tweaks make little difference anyway, best use the default settings!