Well, I read all the replies and agree with each and every one of them except Novell. I'm not against novell but I just don't trust them anymore. They're still faster than windows for a network file system.
Here's some options I'd propose for such a general request. If money is a factor please say so.
Option1: Redundancy, throughput and recovery. I'd put a pair of Win2003 servers together on an inexpensive desktop platform with mirrored IDE drives. These are strictly directory servers (Domain Controllers) for the AD domain. These servers will manage your users, permissions and I'd throw in redundant print serving because it has such low overhead. These will also provide DNS and DHCP service if you need them. Use software RAID1 and mirror the drives. Next I'd add in a smaller NAS unit, probably HP which you can grow as you need to. NAS will provide the disk space and you can separate the function of file storage away from domain administration. You can probably license this solution, with hardware, for under 10-15K with name brand hardware (Dell or HP.)
Option2: All in one ADS/storage server. You can put together a decent sized file and domain server using a single box from Dell or HP. I'd look towards the HP ML350 or DL380 for a fixed-growth solution. Using 173 gig drives you can put a lof of space for a minimum price (RAID5 of course.) Cost would be around $10K with the windows licensing. (2003 server)
Option3: Don't use windows or linux

Please put aside your bias and spend a moment checking out the new Apple X-Serves. I've been working with one as a demo for about 6 months now and let me say that if it ever catches on I won't be surprised. Take the buggeries of linux and put a usable graphical front end on it, provide security and support and ta-da, you have a server. I'd only *look* at this if you have nothing in place for a domain already. I wouldn't migrate to it from a working system with an investment in staff and talent. Biggest gotcha? Cost. hardware is apple and software is apple and it won't work on intel.
Lastly, put 2003 server on a desktop you have laying around and play with it. Going from NT4 to Active Directory is a big difference. You'll find yourself wondering why, when you already have a dns server, you need to create the same one again
Lastly, a minor note on terminal services. If you have any need of terminal services under win2x, I'd point you towards running win2k vs. win2003. Under win2k, any XP Pro or 2000 pro desktop is considered a licensed client and you don't have to purchase additional TS licenses. Under 2003 it is separately licensed. If this means nothing, just ignore it.
D