Well.. The pixple pipelines are the things that are able to run, and transfer data from the core to the computer. Thats why its so good to have the pipelines. Although, the core clock is importatn. But if you have a high core, and not enough pixel pipelines, you bottle neck yourself.
So basicly what im saying. Is that you need pixple pipelines more then you do clock speeds. And you need more onboard memory then you do interface.