Not quite true for Vista, you can install a version from upgrade media without a product key and update that. This is the only way to do a clean install on Vista with upgrade media.
http://www.winsupersite.com/showcase/winvista_upgrade_clean.asp NOTE: This method has been apparently removed in 7.
Though as gugucom says you can often get SBE for less than upgrade pricing. However the OP was looking at a $45 copy of Vista Home Basic and you guys were suggesting Vista Business 64.
Since the OP didn't say which Mac and for what purpose don't you think that Vista Business 64 might just be the wrong OS for the OP. Home Premium is the mainstream edition of Vista or 7, and should be the recommendation unless there is a reason to go Business/Professional or Ultimate.
Right at this point, like Stridder44, I would recommend Windows 7 over Vista and you can get a decent deal on it from a reputable vendor like NewEgg here
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116752 or
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754 (I am assuming you are in the US).
If your Mac has 4 GB of RAM or more or you might upgrade it soon get the 64 bit edition, if it has less than 4 Gb and will stay that way stick with 32 bit as you won't gain a thing with 64 bit.
B