Im in between Getting the 780ti (if the price goes down on black Friday / cyber Monday), getting a second 2GB GTX 670 Power Edition for SLI, or the 290 / 290x.
I guess the question that I should be asking is, if the 780ti doesn't drop far enough to be within my budget, which configuration would have the closest performance to the 780ti? The second 670 would certainly be the cheapest, although I am willing to spend more if needed.
Note: I have heard of the 290(x) having heat issues as well as general driver problems.
if you have any intention of increasing the resolution, whether by getting a higher res monitor, or increasing the number of monitors, I would get a 780ti.
Otherwise, 2GB is just about enough for playing at even 1440p FOR NOW. as next-gen games release for PC and graphics go up, the only thing I can say for sure is that 2GB is enough for 1080p gaming in the forseeable future. so if you stick to a single 1080p monitor, adding another ggtx 670 is your best bet for increase in performance