If it's for serious business you should consider an ATI firepro or Nvidia Quadro workstation card ....
AFAIK, AMD does OpenCL better than NVIDIA, and OpenCL is what Photoshop uses. The green team is a latecomer to that game, and chose to spend their engineering resources pursuing CUDA. Photoshop does not use CUDA. Some photoshop plug-ins do use CUDA, but they will specifically tell you they use it. Like with workstation GPUs, if you're not sure you need, then you don't need it.
Personal experience tells me the HD4890 from AMD is more than powerful enough in Photoshop CS4, and benchmarks say the 6850 is almost twice as fast ($175 on newegg).
Edited by willis888 - 2/24/11 at 8:48am