Since I never bother to read any of the crap that comes in the box, I didn’t know about this, but apparently if you have purchased a retail copy of Vista (including upgrades) you can get the 64 bit version from Microsoft for just under $10. There was a long list of countries to ship to, so I don’t think this is a US only thing.
The website to get this is:
http://www.microsoft.com/windowsvista/1033/ordermedia/default.mspx
(And before the Linux bait starts to drop, I’m looking at Ubuntu as well, but this will let me compare apples to apples)