Question 1: Would this be overkill in terms of wattage? Perhaps I can save $50 and go with two 1000w? And how should I connect everything up.. 4 cards on one psu and the system + 2 cards on the other?
You'll want to use a PSU calculator to determine how much approximate power you'll be need.
http://www.pcpower.com/technology/power_usage/ shows max ranges, most of these are pretty high. Here's a good link to get an idea of the Power Consumption of 280x cards
http://www.overclock.net/t/1454426/r9-270-underrated-the-holy-grail-of-mining-rigs-2-important-metrics.
These are very different in results.Also they dont consider things like everything underclocked except overclocked gpu and that gpu will use more power than in gaming.Since there is no info on net then you just have to find it yourself with a ACCURATE powermeter.