according to benchmarks this week. the ati 1900 has about the same performance as the 4500.

in my experience the 4500 card did not add anything.
whereas the difference between the 6600 and the 7800 was between night and day.
7800 clearly and significantly outperformed 6600 on even simple things.

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how this pans out on macpro's remains to be seen.
no conclusion in the future should be drawn from past experiences.