As you may know, GTX 680 and GTX 670 cards are based on same chip (later has one block turned off).

And it has harware H.264 encoder. No, not CUDA based.

All Kepler GPUs also incorporate a new hardware-based H.264 video encoder, NVENC. Prior to the introduction of Kepler, video encoding on previous GeForce products was handled by encode software running on the GPU’s array of CUDA Cores. While the CUDA Cores were able to deliver tremendous performance speedups compared to CPU-based encoding, one downside of using these high-speed processor cores to process video encoding was increased power consumption. By using specialized circuitry for H.264 encoding, the NVENC hardware encoder in Kepler is almost four times faster than our previous CUDA-based encoder while consuming much less power.

It is important to note that an application can choose to encode using both NVENC hardware and NVIDIA’s legacy CUDA encoder in parallel, without negatively affecting each other. However, some video pre-processing algorithms may require CUDA, and this will result in reduced performance from the CUDA encoder since the available CUDA Cores will be shared by the encoder and pre-processor. NVENC provides the following:

• Can encode full HD resolution (1080p) videos up to 8x faster than real-time. For example, in high performance mode, encoding of a 16 minute long 1080p, 30 fps video will take approximately 2 minutes.
• Support for H.264 Base, Main, and High Profile Level 4.1 (same as Blu-ray standard)
• Supports MVC (Multiview Video Coding) for stereoscopic video—an extension of H.264 which is used for Blu-ray 3D.
• Up to 4096x4096 encode

Via: http://www.geforce.com/Active/en_US/en_US/pdf/GeForce-GTX-680-Whitepaper-FINAL.pdf

It could be very interesting to see native support in main NLE for this feature.

Some tests: http://www.guru3d.com/article/geforce-gtx-680-review/6