I am not too familiar with such a vast implementation of a network, but I can give you a tip or two. With my experience and from what I hear from other Linux users, you probably would want to lean more towards Suse. Novell recently purchased Suse and they know about servers and networks. I hear Fedora is also another good distribution, but I would personally lean more to Suse. My actual choice of distribution is Mandrake, which I recommend to check out. It is the ABSOLUTE BEST distribution for the newbie. Everything in Mandrake is so simple and convenient and they offer support with a purchase of the distribution. They also offer server packs that would easily suit your needs, but they have a price. The Mandrake web site is located at www.linux-mandrake.com
and it will provide you with all the info. you need about Mandrake. As far as free distributions are concerned, they may not be able to suit your needs for such a vast number of users. You may have to buy a Professional version (Suse) or a PowerPack (Mandrake). The cool thing about buying a Linux distribution is that you only need to buy one and not one for each processor like Microsoft robs you for. The Mandrake PowerPack is priced at about $85, the Mandrake PowerPack+ (best in your situation) is priced at about $230 (still less than Win XP Pro and MUCH, MUCH more robust with literally thousands of programs (a list is available on their web site)), and Suse Professional, which is equivalent to Mandrake PowerPack, is priced at about $90. I hope I have helped you in a decision to Linux. I hate to see anyone being robbed by a monopoly that doesn't even need the money. Good Luck!