What computer to buy for PrPro 2015? More GPU than RAM?
My setup: I am running a GTX 760 4gb RAM, i7 3930k overclocked to 4.2ghz (very stable), Sabertooth x79, 800 watt corsair PSU

I once read a really good article and I wish I could find it again to link it, but it basically talked about bottlenecks in an editing system and was supported by actual builds and real world tests. But the just of it was, that unless you are running dual Xenon processors with 64gb RAM the added benefits of a 900+ nvidia gpu was marginal at best. The recommend specs were thus an i7, around 16-32 gb of RAM, SSD's, and GTX 700 series. 

This setup works pretty well for me, and I can recommend it. Instead of forking out extra money for the 900 series GPU, get one with more memory and more CUDA cores (something PP really utilizes especially with 3rd party plugins) in the 700 series. Cap the RAM at 32gb unless your going Xenon. But at that point your going to need a GeForce Titan to remove the bottleneck.

I have found that the mercury playback engine with cuda is considerably better at processing frames especially with effects, than the 'program mode' processing.

For reference I can play 85mbps DNX 1080p at full ress with magic bullet looks on with no lag. If I'm running of an external hdd, it takes some time to start the play head once its stopped (less than a second, but enough to annoy). SSD's are a must.