It would be better if you could choose nvidia instead of AMD due to extensive support of CUDA libraries for image rendering/implementation. If in Laptop, I recommend newly released maxwell mobile series 860M for light purpose uses
I always recommend desktop graphics cards depending on budget , I strongly suggest to wait 2-3 months for release of Geforce 880 series with major architectural improvements along with CUDA6 support
Generally, something what has a good performance for gaming will have a good performance for GPGPU. Image registration is more memory intesive that compute-intensive (with comparison to gaming). Therefore, compare not only computing power (FLOPS ~ #cores * frequency) but also bandwidth (GB/s ~ #bits * DDR5 frequency and omit DDR3 and shared memory GPUs).
GPU branding/naming is a quite mess especially in laptop domain. Focus on raw parameters, architecture (Fermi, Kepler, Maxwell), and manufacturing process.