I might need an OS on a PC which only has Linux. I have seen full Windows 7 packages which have 32 bit and 64 bit disks. Can I put 64 bit on one PC (which currently has Vista on it) and the 32 bit on the other PC (which if bought, would only have Linux on it) ?

Also, what is this difference between full installation and ones where you can only upgrade ? How do I make sure what I get is the right one ?

Also, I've never changed an OS before, anything critical to think about ? Running 2 HDs in the PC but could easily take one out.

You only have one license so only one legal installation. The full you can change computers and do not have to have an original Windows installed. The upgrade you are expected to have a version of Windows to upgrade. In Thailand the normal alternative to full install is OEM. With this you can do a clean install on any machine but you can not easily change to another computer (it is designed for computer builders but anyone can use). As operating system life is now about the same as computer life believe the OEM, at a much lower cost, is the best value in Thailand. Be sure version (language) and bit (32 or 64) is what you want. I still consider 64 the next level as almost nothing currently is available to take advantage of it and there are serious driver issues involved for older equipment.

On the change from Linux believe you will have to reformat the hard drive.