Coincidentally I was looking into this the other day. I don't know that any ATX spec. motherboard will work. The Pentium M is designed for laptops. Therefore the voltages may be different than those for ATX CPUs.
One thing that I didn't do was to look at the Intel web site to find out if a Pentium M would work with the ATX spec. motherboards. The idea of making a desktop machine with a low power CPU is interesting.
Here are the steps that I took.
Go to a price comparison web service.
Search for Pentium+M.
Find an entry for a CPU for sale. Most of the search hits will be for complete notebook computers but there are some retailers that sell the Pentium M CPU in a retail box set.
Go to the retail web sites and find the technical data / specifications.
Find the CPU socket type.
Look for motherboards that use that socket.
Here is one of the better hits.
You will notice that it says that the Pentium M 760 uses a socket 478 PPGA. When I looked for motherboards on the same site I found plenty, but I wasn't sure if they are exactly right for this CPU. The PPGA part of the socket type made me wonder if it was the same as the socket 478 on the ATX motherboards.
Anyway, you will notice that the price of this processor is quite high compared to 'normal' CPUs of the same horsepower. The difference in price would probably never be saved in the difference in electricity that the CPU uses, so that's where I stopped looking. This particular page says that this model of Pentium M uses 27 watts.
If you are still interested in low power CPUs I can tell you that AMD makes a high efficiency (HE) model of several of its CPUs. These CPUs use 55 watts compared to the normal processor of the same model, which use 95 watts. Once again the price of the low power CPUs is quite high compared to the price of the comparable high power CPUs. You would never save money overall by the cost of the electricity that the low power CPUs use, but you would be a good citizen of Earth by doing so.
There is one other possibility for low power CPUs. VIA is making very low power CPUs for the itx, micro itx, and nano itx motherboards. They have a 1 GHz dual core processor that uses 14 watts!! More information can be found at the VIA web site. Look for nano itx motherboards. You will eventually find their Luke processor. Or you can just look here: