Well, you have many to choose from. (BTW, I'm not a newbie to Linux like it says under my name, just to this site, although I wouldn't call myself a power user yet)
Debian is very very very stable as far as I can tell from my experience with it. I used it for a month and other than programs I installed myself that had minor crashes, I didn't have a single crash with it. Do not bother downloading the Woody disks for it though if you want to use Debian. Go to www.debian.org
and go to network install from the left and download the official network install from the unstable or testing tree, that way you only have to burn 1 cd. The rest is downloaded as you install.
I find Mandrake is not as stable as Fedora/Redhat/Debian/SuSE, I would recommend it more for desktop use....although many people have gotten servers running with it, that's just my opinion. Don't buy anything from them though, their customer service is absolutly pathetic when compared to that of SuSE. I was trying to get a refund from them once for this MandrakeClub thing that I didnt't want anymore and it took them over a month to respond to numerous emails I sent to them. SuSE and Linspire both have awesome customer service, they respond within hours (Don't try to use Linspire on a server).
I currently use Fedora and run a home network, not a real server though, and it is incredibly stable, it is like Debian, it never seems to crash. All my computers run Fedora Core 2 and it works great, it is another one I would recommend for your server. You could also use RedHat 7.3 or 9, as they both still get security updates if you alter some files so that it updates from the Fedora Legacy servers. (Fedora Legacy is a project to extend the life of RedHat's obsolete products so that the end of lives are delayed for up to a year and a half so that people don't have to upgrade.)
I used SuSE before Fedora and I absolutly loved them, the only reason I switched is because twice I have mail ordered a boxed set from Novell and it has come damaged. I don't like our mail service where I live and I would rather buy a download than buy a boxed set and Novell doesn't allow that (only one store in Calgary sells SuSE and they are always 2 versions out of date), so if you wanna use SuSE 9.1, you can buy a boxed set or order cheap CD's from linuxcd.org or other sites like that. SuSE is also stable and comes with loads of programs and server tools.
Personally, for servers/networks, I would recommend SuSE or Debian the most out of the list above. and then Fedora and Mandrake last. Anyways, I won't drag on any more now :-P. ttyl..