Extremely complex question, answer depends on a lot of things.what is the best card to get without any modifications to psu etc.
Please look at the first post of the thread below and ask any questions there:
GPU Compatibility List for cMP
Content: 1. Summary 2. Information on common GPUs that can be used in cMP 3. GPU power consumption 4. Color compatibility 5. Flashing nuances 6. What to choose? 7. Useful links. 1. Summary 1.1. There are 50+ GPU types that are compatible with cMP (classic MacPro = Mac Pro 1.1-5.1/...
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