The cards that definitely support OpenCL 2.0 are 7790/R7 260/R7 260X, R9 285, and R9 290/290X.
As long as you don't have huge amounts of data to crunch, I would go with 7790 and R7 260/260X. Those are excellent GCN 1.1-based entry cards. They are fairly powerful already. I develop OpenCL programs for academic research, and have had a few interesting experiences with OpenCL across different platforms and hardware. I never needed anything more powerful than a 7790.
Beware that AMD's early 14.xx Linux drivers have a funny memory leak problem. If you use CodeXL to monitor GPU usage, it always complains about that. I don't know if this has been fixed - it's not a general problem either since the only negative effect I've noticed is slower data transfer rate. NVidia's opencl driver, on the other hand, is very robust and stable, but only supports OpenCL 1.1.
Under Windows, this memory leak problem doesn't exist. All my opencl programs also run marginally faster as a result.
Hope this this helpful!