You don't need the fastest card (GTX680) for CUDA acceleration, a GTX560 Ti 448 (or a GTX570) for example will be quite sufficient. I personally would go for the 7970 for the raw computing performance which seems to be significantly greater than the GTX680. OpenCL seems like it'll be supported in a number of applications released in the future that are currently using CUDA. A GTX560 Ti/GTX570 as a secondary card for specific CUDA applications.
Here's a nice site btw for CUDA acceleration performance:
Adobe Premiere CS5, CS5.5 and CS6 Video Cards with CUDA Acceleration Mercury Playback Unlock Enable MPE Hack Mod Tip
You can see the GTX570 is barely slower than the GTX680.
Heck, even a GTX550Ti/GTX560 non-Ti is sufficient. Could've grabbed a vanilla GTX560 for $140 last week.