I've just ordered an i5 2500K bundle from Scan overclocked from 3.3GHz to 4.7GHz (though with 8GB RAM I might not get quite that)
Anyway, all good questions.
And that is your answer: Use the Blackmagic to feed an HDTV via its HDMI out. This will give you the best possible preview, and free up the whole of your PC screen for your NLE interface.
Just a thought - check your NLE allows previewing to an external device, before doing this!
More important is to keep your OS/Programs on one drive and your video on another.
Scan.co.uk: Antec Solo Black/ Silver Mid Tower Computer Case - SOLO EU which I've had for three years now. The plastic covers for the on/off & reset switches on the front cover (which cover the actual microswitches) are a total disgrace and I've had to glue/tape them up on at least two occasions, but apart from this the case is solid and well acoustically damped (without being OTT).
Make sure you use 120mm fans (they run much slower than smaller ones, use less energy and make less noise)
The difference in cost was too much for me. My stuff isn't time critical so an extra few minutes on a render matters little. What matters to me is a nice silky smooth editing environment.
I should get that - especially as I only work in Standard Def - but we'll have to wait a week or so to find out.