If you're going to install W7 onto one machine then a standard Home Premium upgrade will be fine, it will upgrade XP or Vista. If you have more than one machine then the Home Premium Upgrade Family Pack would be a good purchase as it allows you to upgrade 3 machines.
A retail Version can be installed onto any machine, if you have it installed on one computer then buy another without an OS you can install your retail version to the new one, as long as you uninstall it from the original, it can only be used by one machine.
With an O.E.M. version legally you would have to buy another licence for the new computer as it's only supposed to be used for the original computer it was installed to.
I'm sure I'll be corrected if I'm wrong about this, but I'm pretty sure I'm right.