Testing GeeXboX
Just downloaded and burned the 7 MB GeeXboX iso. I tested it in my 700 MHZ, 512 MB machine it booted really quick into RAM then I proceeded to test it's capabilities of playing DVD and videos on my HDD. It worked pretty well no doubt the faster your machine and the more RAM you got the better that the performance will be.
I am impress with GeeXboX so far and it is free software, open-source software.
http://www.geexbox.org/en/index.html