Stick with XP Professional for SBS 2003 Domains. The setup and operation is VERY dependent on Group Policy implementation. If you have one tiny problem with workarounds, you'll spend a LOT more money trying to fix it than it'd cost to just upgrade to XP Professional in the first place.
I've got one client with lots of "Best Buy" points. They insist on buying HP computers from Best Buy, with XP Home or MCE 2005 installed. I tell them to buy Dells with XP Professional and MS Office already installed. Every time a new PC comes in, I have to buy XP Professional for them and install it. That costs a LOT more than it would cost to just get the correctly-configured Dell in the first place.