Assuming that you are using IIS(if using apache, this info is not of any use to you, you may want to consider apache, although not if youare going to use asp with vbscript or anything). Most likely XP will be fine for you, and you will know when you grow out of it. No reason to spend all the extra cash if you do not need to. Having said that, here is some info and a link.

Some of the limitations of IIS on windows XP pro are that it only allows 10 simultaneous connections, and will only allow you to host one "site" on the box. Windows 2003 server will allow pretty much as many simultaneous connections as you want, and as many different sites as you want.
Of course a site is somewhat blurry, if you just give the people you are hosting a directory under the default web site, it might not affect you.

Also there are several other things which are different. I suspect that you cannot use virtual hosts in IIS for XP. User management and security/policy management is most likely easier done on windows 2003 server. windows 2000 server(2003 as well, I have not looked) also has IISLockdown and urlscan tools, which if run will significantly improve the security of your server, I do not know for sure, but I doubt those will be applicable for IIS on XP(like I said, I havent tried it).

http://www.microsoft.com/WindowsXP/p...rviews/iis.asp