If you are going to go RAID5, I'd rather go hardware RAID5. If you are going to be cheap about RAID5, why spend nearly $1000 on the OS? (Unless you are going to pirate the OS, in which case flush your support argument right down the toilet)
better multiprocessor support?
Better? No. More? Yes. But then, if you are using a machine with 4 or more CPUs, you should be running a server.
more controll over applications
Eh? What kind of control? Like limiting what can and can not run? I can do this with Windows XP.
As was mentioned, Windows 2003 Server is essentially Windows XP pre-patched. Therefor it will be just as stable as XP, assuming you properly patch XP.
better documentation and support through microsoft
Unless you have some service agreement with Microsoft. support via phone/email/website/whatever will be the same. And, in fact, Microsoft might not even bother to offer you support if you use a Server on the desktop, as it may (I don't have access to, nor would I read, for the sake of this post, the EULA) violate the terms of the EULA.
\Dan