The mobile 2500+ will overclock with out and voltage increases beyond that of a regular 3200+. Chances are it can do 3200+ speeds with less voltage than that of a stock 3200+ (less voltage = less heat).
You are correct, AMD has not and will not upgrade the XP (socket A) platform any more. They are still good budget systems, but if you want to piecemeal upgrade (a piece here, a piece there) you will need to get a socket 939 mobo. Even socket 754 has topped out its upgrade path at the 3700+. Maybe a sempron may come out faster than that for socket 754 later, but I doubt it.
You won't need new ram if you were to upgrade from an XP system, but you will need a new mobo/cpu, as long as you bought pc3200 ram in the first place. This assumes that AMD doesn't jump on the DDR2 or DDR3 bandwagon any time soon. (I personally doubt they'll do DDR2, but DDR3 might be AMD's next step - just a guess though)
And, IMO, if you're doing the piecemeal upgrade path, then hold out on a new mobo until the nForce 4 & Pci-express mobos for 939 come out so you can have somewhere to go with your video card. Of course, you won't be able to use any AGP cards any more then...
This is why I recommended getting an athlon64. AMD are phasing out socket A so there will be no chance for future upgrades unless you upgrade CPU, motherboard (and probably graphics card thanks to PCI-e).
I really think a socket 939Athlon64 3000+ and a socket 939 motherboard would be a better combination.
Its better to spend a little more money and know you wont have to upgrade for a while.