In practice the 'system builder' or OEM version is simply a discounted version with less packaging, and no support. Its meant for sale with new PCs, and yes, you can install it on a new PC (and only a new PC as per licence).
Retail is slightly more expensive, and you can install it on anything system, new build or existing pc as per the licence. There is no longer a retail version, only a personal use licence that works with the system builder version
Both are the same software sold through different retail channels - think of one as a 'wholesale' version and the other is 'retail' but they're both the same OS. You can get a System Builder Windows 8 professional copy or upgrade. The only change here is that previously you could not transfer a system builder licence between your own systems, now you can. This would not effect you in any way, and brings this licence in line with the retail licence
As this is a new build, the system builder licence would be a better choice since its lower cost, and you can get a professional edition with that licence. Retail editions would work identically, and just have a nicer box.
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It should be noted that the personal use licence is NOT the same as the OEM licence. The personal use licence has more or less the same terms as the previous retail licences, especially in transferral of licence and support from Microsoft.
– Bob – 2012-10-28T17:21:48.630@Bob - There isn't a personal license. – Ramhound – 2012-10-29T19:55:38.587
2@Ramhound What are you on about? I linked to a licence called the "Personal Use License", hosted by Microsoft themselves. – Bob – 2012-10-29T19:58:28.720