One of the main benefits of W2k3 over W2K for me is the improved security functionality.
The "safer" group policies which are available to you can allow you to lock down specific applications to run at only "restricted" user level as opposed to administrator - if you are working in a corporate environment where the standard response to an app not working as a domain user is to up the user to local admin, you can still restrict IE and Outlook to run with reduced priviledges (this is available in XP SP2, as well, but you can't actually apply the group policy on it from a W2K server - even with a custom .adm file, at least, not so far as I could see...)
The improved windows firewall is also a benefit, though if you need something like that, you can always add one to w2k server.
2k3 is also reduced in functionality from out of the box - rather than giving you an install that has a wider surface area of attack, it does nothing that you don't ask it to - allowing for less chance of misconfiguration.
It's also got some nice add-ins for load balancing and the like - but that's only really useful if you're using it in a large network.
Personally, if I was going to buy a server now, I'd buy w2k3 - if I had a fully installed and working w2k, I'd stick with that.
On the other hand - depending on the needs of the organisation, the server and the budget, I'd be trying out a few of the linux distributions, because in terms of useability (once you've actually managed to install them, which can be a nightmare with drivers) they aren't far off the usability of Windows servers these days - though you'll have a steep learning curve...