Any motherboard labeled "micro-ATX" should fit into any micro-ATX case.
No price checks are allowed on these forums. You'll have to research that yourself. I don't think an Athlon XP is worth much though, probably less than $10.
The primary problem with your delimma, is that socket A/462 is horribly obsolete, so what few scraps you might be able to find are going to be overpriced, unless you can luck out on craigslist or something.
Therefore, going with something a bit more modern is a good idea. But you will have to pay for some DDR2 memory, and probably sell your DDR.
Most G31 and 785G mobos will still have a single PATA port, so you could re-use one CD/DVD and one PATA HD.
One alternative possibility is a board I saw mentioned recently at 3Btech, let me see if I can find it again. Here:
http://3btech.net/as77recoduo7.html
Takes DDR1 memory, and supports Core2Duo CPUs. Note that in order to use the onboard video, you have to use an 800FSB CPU. So that means, an E2000 series CPU for the most part. Perhaps you will be able to find on on the FS/FT forum for cheap. You could also use a Pentium LGA775 CPU (not Socket 478).
