Nothing is installed on your system - activation is recorded as a numeric hardware signature in MS Servers. On OEM the mobo component cannot change without special exception during phone reactivation.
The best deal right now is Home Premium Upgrade Family 3 pack for $125 on buy.com.
That's 3 licenses for less than the price of one. You can share them with family or friends as long as you make sure they are not installed on more than 3 computers at the same time.
Any retail Windows 7 besides OEM can migrate to the machine of your choice.
In the case of upgrade version you need to have a qualifying XP or Vista which came on the computer or a retail copy on hand which doesn't need to be installed since this is on the honor System.
However if you install Upgrade version to a HD with no OS on it it requires leaving the Product Key blank until after install to do the quick registry workaround given here to activate Upgrade version on cleaned HD: Clean Install with a Upgrade Windows 7 Version