If you're looking for max performance over max bang-for-buck then don't waste your money buying anything until the R9 290x is benchmarked. Having 2 very powerful GPUs over 4 less powerful ones might lose you a little bit of top end FPS and average FPS but you will get better minimums due to decreased CPU load and coordination overhead, and more consistent framerates because of the anti-stutter driver (AMD) or generally better stutter control (nVidia). Especially since you aren't running the newest CPU I'd try to keep it to 2 GPUs for scaling purposes. Further, 2 of the strongest gpu gives you the ability to still game decently in the first few days of a game where there are no Crossfire/SLI profiles out, or on games that suck at scaling. FYI: the Frame Pacing driver for AMD does not support Crossfire X at this point so I'd rather buy something that I know works rather than rely on promises that it will eventually work. And then you can add a third later if you really want to down the line if and when they do actually fix it (and a non-issue if you go with 780s)
I wouldn't go any less than 2x 780 or 2x R9 290x if that ends up being better/cheaper for same speed. The firesale prices will probably last until at least when the new cards come out. And if it does well, nVidia might drop pricing on the 780. Honestly if you still have money burning a hole in your pocket, you'd probably see better gains by getting the top 2 cards and getting watercooling for them to push their clocks as far as possible since the 780s OC well and the general anticipation is that the 290x will probably overclock decently too