What are the downsides to buying the OEM version of an operating system?

I recently moved and in the process of moving, it looks like the hard drive with an OEM version of Win 7, Enterprise took a jolt. It appears to have suffered physical damage to the boot sector. To make matters more interesting, I’ve searched high and low after the move, and cannot find the CD of Windows 7. And wouldn’t you know it, the only place I had the software key written down was in an Excel file on that drive. Lesson learned in backup to an external drive as well as off-site in the cloud.

I’ll have as many as 5 machines on the physical router network including two desktops (one of mine and one for my wife), two tablets and my notebook. I may link in hand-held devices by wireless or Bluetooth. Being able to plug in multiple displays to a single machine would be a big plus in Web Development.

I have two new 1 tB drives to slap into my tower machine, but they aren’t much good without an Operating System. Since it’s truly a box I built up from motherboard on, what is the downside to buying the OEM version of the software? A copy of Win 7 Professional or Ultimate 64 bit Complete is about $300 to 320 respectively. The OEM versions are anywhere from $80 to $100 depending on vendor.

I think I will avoid Enterprise this time to better steer clear of problems with Micro$oft. I do not have a VLSC status. That said, what level of grief might I expect from using an OEM version rather than a consumer Complete Installation package? I don’t want to go in a direction that will shut me off from upgrade and patch downloads from Microsoft.