Ubuntu Studio. It literally contains _everything_ in terms of multimedia
open source software.
You can either get a DVD with the Studio installer, or just get the
plain Ubuntu 7.10 CD, install it, then install Studio with the method
described in the second link:
http://www.ubuntu.com/getubuntu/download
https://help.ubuntu.com/community/UbuntuStudio/UpgradingFromGutsy
There's no way to tell what CPU speed you will need. It depends a lot on
what you're actually doing. The best way to find out is to actually run
software on that machine.
In terms of audio interface, you can use a cheap common card such as a
SoundBlaster. Or, if you need more channels, the M-Audio are well
supported and work pretty well. I've an M-Audio Delta 1010 and I like it.
http://www.m-audio.com/index.php?do=products.list&ID=pciinterfaces
--
Florin Andrei
http://florin.myip.org/