Windows Vista OEM vs. Retail

[stargate125645]

So I have a 4870X2 and a V2400W in transit to me. I also own a copy of Windows XP Professional 32-bit and 64-bit, as well as Vista Ultimate 32-bit. I want to use Vista Ultimate 64-bit so that I can fully utilize my 4GB of RAM as well as DX10. As a person who does custom builds, I will naturally have to do clean reinstalls of Windows periodically, though the copy of Windows will only ever be on one machine at a time. Given this information, I am hoping some of you can shed light onto the following questions.

What is my best (and preferably cheapest) solution to get Vista Ultimate 64-bit? I see that the OEM DVD on Newegg is cheaper, but it says it is for system builders only (which I can qualify for), but the whole 1 install thing I hear about has me worried. Would I not be allowed to reinstall Windows on any new builds, even if that Windows OEM version would only ever be installed on one computer at a time?

Assuming the OEM option isn't viable, the next cheapest solution is to get the Upgrade option from Microsoft. What are my limitations by using the Windows Vista Ultimate 64-bit Upgrade version? I can apparently only buy it from the Microsoft website, but it's still cheaper than the full retail version of Vista Ultimate 64-bit.

Thanks in advance!