olant,
I don't know if there is a performance difference between Ultimate and Business, but I assume that it would depend on what features you have turned on and how you use it.
You might look at this link and see the differences between Business and Ultimate. Ultimate has all the features of Business and more. You could just turn the one you do not want off. Ultimate comes with 64 bit and 32 bit. You just choose which one you want to install and the same Product ID number for it that you are currently using. I doubt that the retailer will take Ultimate back after you opened it.
http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx
Hope this helps,
Shawn