so i got vouchers and stuff for xmas and birthday etc and im looking at getting another AMD R9 270 and crossfiring on them.

I went to double check at my local tech shop, but i dont trust them all THAT much, their oppinions often cost me more money than is necessary.

1. they posed me the question, does my motherboard support it.
I have the MSI Z87-G43 gaming, which apparently does have crossfire support and i can see a second PCI slot, so im pretty confident it will support the second R9 270, but I just wanted confirmation from those who know more about the technicalities than me.



2. my second question regards power consumption and makes me pretty nervous.

I have an i5 4690k, not overclocked (i know, its a waste considering i have a motherboard thats encourages overclocking and a cpu that encourages overclocking, but my tech knowledge is pretty average, so stuff like that makes me nervous.)
1 SSD
2 HDD
1 R9 270
2x4gb 1866mhz DDR3 RAM

so I did the power consumption calculator thing from here http://www.extreme.outervision.com/PSUEngine
and it tells me that with the extra R9 270, i need a 540W psu.
I currently have a 550W that does have an extra 6 pin.
While the tech guys i spoke to suggested a 700W

Ive had power supply problems in the past, so any thoughts on whether to drop the cash on a new 700W or stick with the 550W would be greatly appreciated.

3. crossfiring is new to me, so is there anything i need to know about it?
if im dual screening, do i need to plug one monitor into one graphics card and the other monitor into the other graphics card?
anything i need to do in terms of setup and software/drivers etc?
any thoughts on anything?

4. do i need to upgrade my RAM to 16gb? would it make a noticable difference? I can afford it, but not sure if I wanna drop more cash on this.