I recommend that you try the Live-CD version of Agnula if its still available. If both distros dont load, you probably have some hardware that needs work. Best supported cards are RME and Maudio. (And soundblaster Live/EMU10K1-cards).
A great place for info is
http://www.linux-sound.org/
At the moment im stuck with windows, as I am working on old projects utilizing Reason (I do not make Reason-music, I just used the samples for sketching Ideas). Ive tried many distros, but used Agnula the most. When Im done in windows, I hope that Ubuntustudio is out. If not, Ill go for
64Studio.
Ive also used Cubase ALOT, and have recorded and mixed with Protools in what is considered good quality studios around here. I cant speak much for stability in the various Linux distros, but the sounds seems to me just as professional, and the graphical interface invites to a more open, flexible and intuitive way of working digital.
My setup is a Athlon64 3000+, 1.5Gb Ram, Maudio Delta 1010Lt. Ive run Protools, Cubase and Ardour smoothly on it. Ive been going over to tracking analog lately, so the DAW becomes more and more a sketching pad. When my (small) studio is done, I will be offering "clients" a DAW with Free Software.
As for performance, much of it is up to your Audio Interface. Maudio and RME is recommended, and PCI is generally more "plug and play" (so Ive heard.) I have run several effects in JackRack, made samples in Specimen, singletracked in Ardour and recorded midi in Rosegarden with a P3 500Mhz, 380Mb Ram and a crappy Soundblaster "Vibra" PCI 128.