What kind of a figures do I need to supply my 3x 1080p needs? Is 192-bit is referring to the Memory Interface? What is a good ball park for this? 256-bit? What about CUDA, how high am I looking at? And vRam? 2GB? 4GB? higher?
When SLI'ng, can I just double the memory interface, vRam, and CUDA to give me a figure? So let's say for example, X game requires 512-bit for 3x1080p and CUDA 2688, then in theory could I just SLI 670 and meet these requirements? I have seen some really good results from SLI of 670 and your right, the 680 isn't that amazing for it's price.
In SLI both cards need their own memory, which means you can’t add them together. Same goes for memory bus.
The question of who has sells the better high end graphic cards is pretty close this year. I would have to say the 6xx are better when you game on a single screen, but on a triple screen setup AMD at least catches up. This is probably because of the faster memory bus and also a little because of the 1gb difference (2gb for nvidia and 3gb for AMD). Besides that AMD cards overclock a little better, partly because the stock clocks are so conservative. Driver wise I would give the edge to nvidia, but eyefinity is more mature compared to surround. If you want to run either two gpu’s SLI is the way to go. Especially because SLI setups produce less microstuttering. However running two gpu’s lets you turn up graphical settings, which means you need more memory. So although SLI seems the way to go, almost all 6xx cards have 2gb, so you might run into a memory bandwidth problem (although based on most reviews this is highly unlikely with games that are currently on the market).
There is one thing I don't get, why are you concerned with the number of CUDA cores?