Depend on what you want to use it for.
512MB of RAM will be enough to get a desktop and run normal apps and programs like Audacity; If you want to use Ardour 2GB is recommended. See http://ardour.org/requirements.html KDEnlive (video editor) needs 1GB memory too.

As for sound cards and on board hardware, anything supported by the ALSA drivers will work, and that means the majority of sound hardware.

Again I recommend asking more detailed questions on the AVLinux forum, where you may meet users with similar hardware and more collective technical knowledge than you'll find on Mudcat.

KXStudio comes recommended by some users, by the way, but I've never used it myself.