Your excitement is infectious, that’s a nice piece of hardware!
The best distro is the distro that works for you, I know that sounds like a bullshit answer but it’s very difficult to say because each persons needs differ and each distros focus differs.
Fedora has great support with Planet CCRMA, openSUSE has great support with Jacklab, and Ubuntu has great support with Ubuntu Studio and numerous 3rd Party repos. Debian has MusiX and 64 Studio or Debian Sid for the brave. I’m sure there are others as well.
I learned a valuable lesson myself this year. I had an older but very stable setup with Ubuntu 7.10, Ubuntu 8.04 came along and I thought it might be better to stay current so I upgraded. BAD IDEA. I won’t bore you with all the details but after a month and being tempted back to Windows XP, I finally rolled back to my old 7.10 setup and learned to compile whatever packages I felt were outdated. The benefits of the latest greatest versions are quite often more hype than substance.
A Linux Audio DAW is a little different in that the balancing act between JACK, ALSA, Freebob, Ardour and even Wine is more delicate than in a Windows environment. If you have a stable system that works with your hardware I would strongly advise you to leave it alone. Audio/Video in Linux is at the peripheral edge so most Distros pay little attention to how Upgrades will affect users of those particular applications.
The developer of Cinelerra still uses Fedora 2 or 3 and look at the complexity of that application!
I have opted to keep my Audio distro as my foundation, If I want to install the latest version I either try it on a different machine a different partition or most often in a Virtualbox VM image. That way I can kick the tires on the new version and see if it will continue to suit my A/V needs.
Others may feel free to disagree, but I have truly experienced “If it ain’t broke don’t fix it”
As for your PAM question, simply browse to your /etc/security/limits.conf file and see if the following lines (or similar) are present:
@audio - rtprio 99
@audio - nice -10
@audio - memlock unlimited
If they are there than you are good to go, if not add them to the bottom of your limits.conf and save it