Considering the cost of Windows Server 2003 you'd probably be better off just staying with XP. Unless your gonna be hosting a very high number of people XP should be ok for you.
I use Windows 2003 to host Unerror and it's great, alot more stable than Windows XP, or any of the other Windows versions I've used before. And, let me remind you that Windows XP is not a server OS. Hosting Websites on Windows XP Home is not a good idea at all, it isn't the greatest for stablilty, or speed. I recomend Windows 2003.

EDIT - Well, acctually, I would recomend Linux to host servers, but, Windows 2003 is an amazing Operating System compared to the other versions, in my eyes.