I was looking for similar answers about all this the other week, this link may be of interest,
http://windowshelp.microsoft.com/Windows/en-US/Help/3af7e05f-4d2a-4af7-a168-9242f9093bb81033.mspx
Unsure of the charges (Windows anytime upgrade). There are further links in the bottom of the page which amongst other things tell you the recommended install options of all upgrade paths eg. upgrade or clean install. But as has been said 32bit > 64bit is a clean install only.
Another thing is, not all packs come with 64bit in the box and you have to request it be sent to you from Microsoft - for a so called "minimal fee", see my
thread about it,
Edited by deleted (Mon 11-Jun-07 13:08:51)