I have a Desktop barton 2500+ from before there were mobiles. It overclocks reasonably well, to around 2300 MHz or so.
I recently have been building a low-end system for a friend, and rather than have them buy a super low end processor, I'm giving them the Duron 800 from my backup system and I bought a Fry's special 2500+ for my backup system. That one overclocks to around 2100 MHz. And from what I hear, that is kind of the norm now for a desktop Barton.
For the chips that come from the Barton manufacturing line, there will be a certain amount of variation in the performance capability of the chips. Now the chips that go into the Desktops and the mobiles are not physically different from the manufacturing standpoint, they are different parts of the same population. To be a good mobile, it has to be able to do a given MHz at as low a voltage as possible...
If you look at the voltage ratings for the 35W 2400+ and the 45W 2400+, you'll see that the 35W is at 1.35v and the 45W is 1.45v, this is where the power difference comes from. Presumably the 45W mobiles are not capable of doing 1.8GHz at 1.35 volts, or they would have been binned as a 35W. Now, there will probably be some overlap among the different rated processors to account for demand of the various grades vs. production capability, but in general, this should be true.
The 35W mobiles ability to do 1.8GHz at a lower voltage DIRECTLY translates to a better overclocked processor, as the same internal functional advantages that make it able to do a higher MHz at 1.35v will allow it to do a higher MHz at the default AXP desktop voltage of 1.65v, or at whatever voltage the 'enthusiast' chooses to run. Quite simple really.
This is why the 35W 2400+ are so desired and command the extra few bucks. They are the highest clocked 1.35v AthlonXPs. So they must be pretty near the top of the distribution. The 45W 2400+ mobiles will likely be lower capability, as AMD sells a 2600+ 1.45v mobile processor as well. It's basically pre-sorted to overclock well, even though that was almost certainly not AMDs intent.
The other supreme advantage of the mobiles is that they are unlocked. So you can basically set your FSB however fits you best. The desktop solutions are limited to whatever multiplier you had, all the 'tricks' to unlock them are gone. Bridges and pin-mods don't seem to work anymore.
I mean theoretically a 3200+ should overclock well too, after all, not all the Bartons can reach 2.2 GHz without issue at 1.65v. But in reality, you're locked at 11x, and to get 2.4 GHz, you need to run your FSB to 218. Sounds easy enough, but the selection of motherboard that can do that is somewhat limited. When you start talking about needing 220+ FSB, you can basically forget about it unless you're into volt modding your hardware. Not to mention tha tthe 3200+ is nearly $200.
While a desktop 2800+ will work fine, there is NO reason to buy one when you can get a 45W 2400+ for like 30% cheaper that will overclock the same or better... or for ~20% cheaper you can get a 35W chip and pretty much be guaranteed that it will overclock better.