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.
I've heard of people linking power supplies together so they get turned on at the same time, how do I do this and is this necessary? What do they mean by "do not mix" when doing this.
Organik is right on, you can use a paperclip or even an industrial staple to short the 24 pin, you just insert it into the pin with the green cable, and a black ground pin. You can then setup your BIOS to turn the computer on when you power it on. Otherwise you can use a screwdriver (plastic handle) to short the motherboard to start.
I have 6 USB powered risers.. some of the posts I've read from people make me nervous about this whole power concept. But my plan now is to connect the two molex on the motherboard up, as well as connect all six powered risers to each card. I guess I'm concerned about frying the motherboard or the cards. Hopefully someone can give me some advice here on the plan.
I wouldn't worry about this, the risers are 25W max additional load. The powered risers will pull from the PSU (if I'm not mistaken).
Last thing. Your RAM is overkill unless you plan to game too. 2 GB max, the idea is low power for everything but the GPUs. Use Linux and a USB thumb drive (I use 8 dollar ones). Honestly, I wouldn't got with 280x, but that's up to you.