should clarify a little more...
the functionality of the Mercury Playback Engine is not accelerated by ATI cards, they only are enhanced by CUDA which is only offered with Nvidia cards. so while they work, it's not as huge of an upgrade for those Adobe folks.
I'm sure you'd see a bit of a performance boost with a card that has more on-board memory than the stock 5770/5870 GPU's, but nothing as close to using an Nvidia card that supports CUDA. It's probably only a matter of time before other Adobe applications start tapping into the Mercury Playback Engine style of processing as well.
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I thought that the Adobe suit really likes the NVidia more than Radeon, and the MacBooks also uses GeForce, so why go with Radeon?
yes, Adobe uses CUDA processing for accelerating the Mercury Playback Engine... and that's Nvidia.