I've always found with ATI cards I can still percieve the jaggies unless I turn the AA up one notch above what I use on nVidia :X
I think you've been doing something wrong then. I've been swapping between 4870 back to 8800GTX, then 4870 to GTX 280, from 2x 4870 (512mb) to GTX 295 all in this week and AA has been perfect on both ATI and Nvidia. This has been on i7 and X58, E6850 X48 and this system. 4xAA and 8xAA looks fine on close inspection on both.
This is a new problem to me. I've never heard of this complaint from Nvidia users before. I came from Nvidia with no problems and have swapped cards in my system quite a few times between the camps. I build a lot of friends systems also and I'm constantly swapping ATI to Nvidia or Nvidia to ATI and I've never heard of this from anyone.
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Uninstall drivers, run Driver Sweeper then restart. Safe mode, run Driver Sweeper again, restart. Install drivers, restart. Never had a problem in years doing this through several systems. (I'm not telling you how to do it Rroff, just mentioning what I do as I've never once experienced anything like you are talking about)