The limitations of an OEM license are:
Can only be installed on one computer (the first computer you install it on is the one it is then tied to). You can however, reinstall it on that computer as many times as you want to, or need to. If you buy a new system (Microsoft defines a computer by the motherboard), you must buy a new copy of Windows.
No support from Microsoft (because you are supposed to be selling the computer to someone else, you are the one providing support for it)
When buying the copy of Windows, you must specifically buy the 32 or the 64 bit version (both disks come in the retail version of the product).