Contrary to everyone's answer, I'd have to say GTX 770. Why? PREMIERE PRO ENABLES MPE (MERCURY PLAYBACK ENGINE) WITH A CARD THAT USES CUDA.
MPE allows specific effects to be added to clips without the need to render in order to preview.
You can use OpenCL cards in CS6, but you need to use a "hacK", which seems to be hit-or-miss according to some sources. Support for R9 290 was just added in Premiere Pro CC 7.2.1. In CS6, there are 3 blur effects that MPE isn't covered in OpenCL cards that is in CUDA...
If you use After Effects a lot, it can help when outputting as you can nest your AE project into your PP sequence and use MPE (check use previews).
With just a CPU for MPE I couldn't notice a significant difference with the effects I was using. So I tested by having the "Previews" option in the output window checked and watched how much time it took to output the same video with MPE enabled and disabled. The CPU was anywhere between 1/3-2/3 slower than when MPE was enabled. I did about 4 passes, deleting any present previews each time.
A multi-core unit may make a difference for 3DSMax , I don't recall it being a huge issue when I used it, but I wasn't paying attention either. However, After Effects is a RAM hog! The more RAM the the longer you can preview in better quality, which can make all the difference. If you are dealing with an IDE HDD, then scratch disks may become an issue. Buy two identical IDE HDD's and do a software RAID to fix that. If you have a SATA disk it shouldn't be an issue, but if it is the same trick applies. I don't use my SSD as a scratch disk, but Hybrids would likely run fine.
@jamn805 - If you have already bought your hardware, sorry for the late reply. And if you are in the 805 area code, check out sbfilmmakers.com for a community of filmmakers close to you.
If we were talking just speed, then the R9 290 would take the cake. It will definitely be better for some games in terms of FPS, but I find my 680 runs fine for most games (including Crysis 3). You could get higher resolutions with the 290 as well, but I doubt most will need it for that. I would likely recommend the 290 over the 770 if you were using Premiere CC 2.7.1, but if you are on CS6 the 770 would be a better bet in terms of working. (I have yet to see an article or post about the 290 in CS6, good or bad.) The 770 will also be a lot CHEAPER, save you a bit of power, and in the tests I saw you should get only around 10-15FPS less on games.