like Fellenteee said; you should not use as 12v -> up transformer... the powersupply will only turn that back into 12v and 5v
use one of these instead:
http://www.picco.nl/index.php?cPath=21_ ... f2f645660e
PicoPSU beeing the smallest.
Also i would recommend not to use the 300mHz motherboard and cpu, it's old and therefore burns way to much power. A nice mITX board with onboard cpu is going for like 70e brand new (
link) (NOT good enough for hd video!! but it's only 70 e he

)
I do not know your true intensions but maybe a 8 or 16gb flash drive offers you enough data-storage. This is far cheaper then buying a ssd disk and consumes less power, keeping in mind that you use a fearly modern mobo so it is able to boot from a usb device. It's off course possible to use multiple flashdrives to expand your data-storage capacity.
Futhermore the software.
If you are not going to use a GUI then winamp with the (free) com-port plugin is a nice way to go. A small plugin for winamp which let's you control winamp by using the com or print port (with a small elektrical board, which is VERY easy to build). A set of small buttons wil then give you the ability to paus/resume, next, prev, mute etc...
going with a GUI, you have several options. There are some nice freeware Linux projects; just install and you have a WMC like interface ready to go.
So in short, i would buy myself a dc-dc powersupply, modern mobo and use flash drives as storage (including for booting a operating system)
Keep us posted!
sjors