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Always go OEM if possible IMO, it's usually far cheaper. Apple don't actually recommend OEM for Bootcamp but I've found it works fine.
Buy the one that suits your needs, if all the extra networking stuff etc is worthless to you then don't buy ultimate, home premium is probably the way to go.
 
With OEM, you get no support from Microsoft and legally you are not eligible to use it as the license is only for computer that will be sold i.e. for system builders. However, MS doesn't seem to care about it and you can use the OEM version just fine. The actual software is the same, the only difference is the license key and price.

I always use OEM versions and have never had any issues. OEM serial will only work ONCE while retail version's serial will work several times and it can be installed on another computer using the same license key. Retail version also comes with 32-bit and 64-bit disc while OEM comes with either 32-bit or 64-bit
 
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