One more thought - why not get MCE 2005, and Vista Ultimate at the same time. Buy your copy of MCE 2005 from Newegg, and to save you from having to buy Vista Ultimate as well, you can get it free from here -
http://wfp.microsoft.com/Welcome.aspx
That is if you don't mind them onitoring crashes on your sytem for 3 months, and filling out a couple surveys. At least it saves you $400 if you want. It will also have both 32 bit and 64 bit on the same DVD. You can dual boot both of them to take a look.
Hopefully this helps you make a decision.
-S