As far as the processor goes, it's not that much about the subtype of i7 that you get, since they all utilize the same architecture. Whether you'll get a larger or smaller clock frequency or 4 or 6 cores doesn't really matter that much when all i7's are naturally overclocking themselves when appropriate (I guess); and you can easily overclock them manually. For instance I have a i930 i7, which is currently one of the cheaper ones, but I never registered any kind of problems with it and most of my rendering and regular usage is smooth as it can get.
As for RAM manufacturers I recommend Kingston, and get at least 8GB of RAM, because you're already gonna lose a large portion on the necessities to keep Windows running. Of course, the more you can get, the more resources you got for your editing and previewing can be easier.
One thing I absolutely recommend (it is a thing that I messed up with when I initially built my current rig) is getting at least 40 gigs of SSD. Because if you're slowed down by (much) longer times it takes to read/write from a regular HDD, then it can sometimes be futile to have everything else maxed out.
Anyways, that's it from me. Hope I've been helpful.
