The Intel Media SDK may be of interest to you because its designed to leverage the capabilities of the GPU to perform rapid encoding. In most cases the CPU load is negligible. The encoder link that you referenced is part of the Media SDK Samples.
I recommend downloading the SDK: http://software.intel.com/en-us/articles/vcsource-tools-media-sdk/ and checking out the documentation. Given the few requirements that you listed, I think the Intel Media SDK may be the right direction.