I second this, full cover blocks IMO are much more aesthetically pleasing. and make sure you get yourself some rads that handle low RPM fans so you can have a nice quiet system
I personally own a 120Hz monitor and 780, in your case, i would get another 670 or even 2 more 670s and have some great performance for the price of the 780
ExtremePSUCalculator puts that ~350W which would leave you plenty of headroom for overlocking with the 560. Seasonic PSUs are also notorious for stable 80+ gold performance past their recommend wattage ratings.
I'd say go for it, the scaling may not be ideal but that much GPU performance for $100 can't be beat, and then you can always use it in another system if needed.