This board is super small but it's only a 586 and not upgradeable to a Pentiuim. I guess it will play mp3's ok but that's it. So if you were planning on watching movies or anyhting else it wouldn't work.
If all you want is mp3's and a super small player I would check out http://www.myplace.nu/mp3/.
My friend and I are thinking of building a few of these just to see what we think.
There are others also, this was the only one I could find quickly...