Views on OEM Windows.

I'm going to build a PC and I am doing alot of research, I've chosen all my parts etc. but I was just looking at windows 7 versions and basically came across OEM versions of Windows 7;

http://www.amazon.co.uk/64-bit-Engli...ows+7+ultimate

I don't really need ultimate but since it is so cheap, why not?

I have some questions though, I have read that due to licencing the key is not transferable? So my question is say my HDD failed how would I go about re-installing windows on a new HDD if I'm not allowed to 'transfer' the key? Has anyone had this problem?

Are OEM versions worth it?

Thanks in advance,
Ryan

p.s Sorry if this is the wrong forum, wasn't sure where to put it.