Difficult question to answer at this moment. It depends on a number of things. What kind of material do you edit? If it includes RED 4K or EPIC 5K, I would definitely wait for the 4 GB version, but if you don't do that regularly or only in small amounts then the 2 GB version may suffice. The GTX 680 has of course the advantage of being PCIe-3.0 which can be put to good use on your 2011 platform and supports three or four monitors, something the 580 can't do. Whether the 680 will be faster than the 580 with 3 GB remains to be seen. The number of CUDA cores has tripled, but the memory bus was reduced from 384 to 256 bits, and it is not yet clear what impact that will have on performance.
A second thing to consider is your intended AE use, which you say will be heavy in the future. Tthe fact that After Effects can use multiple GPUs for CUDA computation (for the ray-traced 3D renderer) makes using some GPU setups sensible that may have been a waste for Premiere Pro.
IMO, you currently have the following options, including the announced but not yet available 4 GB versions of the 680:
1. Stick to the 580, but add a 560 Ti for AE and steering a third monitor.
2. Step up to the 680 2 GB for steering a third monitor, but with the risk of running out of VRAM with RED or EPIC material and thus the reduction to software mode only.
3. Step up to the 680 4 GB for steering a third monitor without the risk of software mode fallback.
4. Take a gamble that the 685/690 with a 384 bit memory bus may be announced in the near future (not likely).
The problem is that TSMC, the supplier of the nVidia 28 nm chips has serious difficulty meeting demands for these chips. See http://www.digitimes.com/news/a20120405PD218.html