You've got it right, here's the maths.
Cpu performance wise an i5 2500k would be fine with 2x 680s however SB does not support PCI-e 3.0 and if you need PCI-e 3 you will need an IB or 2011 cpu to support it. This is important to a certain extent. Let me explain PCI-e bandwidth:
The greatest bandwidth is given by
PCI-e 3.0 x16 which delievers
16gb/s
After this the relationships are linear so:
PCI-e 3.0 x8 and
PCI-e 2.0 x16 = 8gb/s
PCI-e 3.0 x4 and
PCI-e 2.0 x8 = 4gb/s
and so on.
The latest generation of cards are not able to saturate an 8 gb/s lane but will fill a 4 gb/s lane resulting in bottlenecking. (I'm unsure about the 7990 and 690 since they are not out yet)
If you take a look at the wiki for the
1155 and
2011 sockets you will see that they have the following lanes available:
x68 = PCI-e 2 x16 (total = 8 gb/s)
z77 = PCI-e 3 x16 (total = 16 gb/s)
x79 = PCI-e 3 x40 (total = 40 gb/s)
This means that if anyone is running a single 680/79xx card then a standard z68 mobo will be fine. However if you sli/xf on a x68 then the bandwidth would drop to 4 gb/s per lane which is potentially limiting the gpus depending on the game.
A z77 board with a IB processor supports PCI-e 3.0 so it's basically two z68s so you can run 2 x8 lanes each feeding 8 gb/s to 2x gpus.
An x79 is therefore only necessary if you are running more than 2 gpus and it will
theorectically handle 5 gpus in sli/xf providing plenty of 8gb/s slots.
So there you have it, if you are running the latest gen gpus:
x68/i5 SB for single gpu.
z77/i5 IB for dual gpus.
x79/lga2011 for more than 2 gpus.
More info here
As for quad channel memory - well that allows you to put more RAM on the mobo, simple as that. Gamers only need 8gb though which all these support. RAM speed is irrelevent to gaming as well.
M&P