I bought an Athlon XP-M 2500+ from NewEgg a few weeks ago and tried it on two different motherboards.
The first one was a Gigabyte GA-7S748-L, which to put it simply didn't work with the Mobile Athlon. The BIOS couldn't recognize the CPU, so it defaulted to a 12x CPU multiplier and a 100MHz FSB speed, and there was no way to change them! I think it also defaulted to a 1.65V vcore, which was too high. The vcore can be increased in 5%-10% intervals (or something like that) but it can't be lowered. (I want to run the CPU at 1.45V or less.) Needless to say, a CPU running at 1200MHz (12x100MHz) and 1.65V just doesn't suit my needs.

I was disappointed, because I was really looking forward to using a motherboard with the SiS 748 chipset and some Patriot "Extreme" memory I got on sale. I couldn't find a lot of information about the motherboard before I bought it, but I read that it had "nice overclocking options", which just isn't true (but see below). Other comments: there's a capacitor next to the CPU socket that makes it nearly impossible to use a CPU cooler whose footprint is larger than the CPU socket (which ruled out the CPU cooler I wanted to use), and it's difficult to set the bus speeds because of the wacky way the BIOS makes you do it (you have to get them in sync). On the bright side, I installed a "normal" Athlon XP 2500+ and overclocked it to 2200MHz (11x200MHz, the same as an Athlon XP 3200+) just by changing the FSB to 200MHz without any further modifications. With an OCZ Gladiator 3 and Arctic Alumina, the temperatures stayed below 50 degrees C most of the time. The motherboard is "pretty" and it looks good in a computer that has a case window, and it performs very well for a single-channel system. The person I sold it to is very happy with it.
The second motherboard (which I'm using now) is a Shuttle AN35(N)-Ultra, which I highly recommend for overclocking. The BIOS doesn't recognize the Mobile Athlon if you deviate from the standard FSB and multiplier settings, but the CPU runs just fine at any setting (within its limits). Here's the nice part: the BIOS lets you adjust the CPU voltage from 1.25V to over 1.85V in 0.025V increments and lets you select any CPU multiplier from 8x to something like 20x. (The default multiplier for the Mobile Athlon is 12x and that's how it will run when you first install the CPU, but you can change it.) With really good air cooling, my CPU can run at 2500MHz at 1.75V, but I haven't tested the stability at that speed and I suspect it isn't stable enough for everyday use. Increasing the voltage does not enable it to run any faster. 2400MHz is probably the upper limit for reliable operation. I'm using 512MB (2x256MB) Corsair "Value Select" PC3200 dual-channel DDR-SDRAM with "Optimal" settings in the BIOS. The Shuttle AN35(N)-Ultra could probably be called a "Budget" or "Value" motherboard, and it's a bit skimpy on features (no RAID, no FireWire, no built-in PC speaker, only has the MCP SouthBridge, etc.), but the performance is great, and I don't really need those features anyway.
Some of you might think I'm crazy, but I don't want to run my CPU as fast as it can go all the time. (I don't drive my car as fast as it can go all the time, either.)

My goal is to have a cool, quiet computer that has decent performance. A Mobile Athlon (with an unlocked multiplier) is perfect for me. When I'm just surfing the internet or doing other simple stuff, I run my computer at 1600MHz (8x200MHz) at 1.325V (yes, that's about 10% under spec). My Antec 300W "Smart" power supply is virtually silent; the fan runs at about 1300-1500rpm nearly all the time, and any noise it produces is drowned out by my Hitachi 160GB hard drive. (I guess I need a Seagate hard drive if I want my system to be any quieter.)

I'm using the heatsink from a ThermalTake Volcano 9 CPU cooler with Arctic Silver 5 (I finally got some, and yes, it's great), but I replaced the fan with an AeroCool Aluminum LED fan that runs at 2000rpm, which is pretty quiet (<20dB). I don't have any other fans in my computer; my Radeon 9000 has a heatsink but no fan. The light air flow that comes out of the back of my power supply is warm, but it's better than the roaring space heater I used to have.
When I want to play a CPU-intensive game or crunch a lot of numbers, I crank it up to 2200MHz (11x200MHz) at 1.6V, which is "Prime95 stable". I don't have exact temperature readings under load, but the temperature stays below 60 degrees C at all times. (Keep in mind that I'm not using the most efficient CPU cooler due to my concern about noise, so my temperature readings shouldn't be compared to most of yours.) I adjusted the BIOS to shut down the computer if the CPU temperature ever reaches 60 degrees C, and it has never shut down on me, so I think I'm well within the "safe range". (If I remember my figures correctly, Mobile Athlons can be run at up to 100 degrees C.)
The overclocking potential of my CPU appears to level off pretty fast at 2400-2500MHz. It can make it to those speeds, but the stability is questionable, and I don't think it will go any higher than that. People who have CPUs that can reach 2600-2700MHz with air cooling are very lucky.
