I've tried about 18 cards in the last year... anything that has 512mb ddr2 will probably do well, GPU core considered.
If you do a lot of large models, with glass and Utilities (sinks, toilets, rounded GSM objects) and tree models (we're talking a lot of polygons) then the FASTER the card, the better.
I've found that the consumer cards (I.E. cheaper) work great. AC does NOT need much to run at all... my 8400 GS in my portable laptop (1240x800) runs fine, but stutters on big models. The 8600 GT can handle anything so far. My 7900 GT/GS and GTX's can handle everything. The fire x1700 and up can do anything, as well as anything in the professional realm (but you don't need a pro card for AC!!!!).
If you can find a decent Geforce 8800 GT, you'll be very happy. Also the ATI 3850's are very cheap and run great too- but not quite as fast in 3d as the 8800's.
AC will run on my old ATI 8500 Radeon Pro's, and my 9700's no problem- and that's with 128mb !!! 10 story commercial buildings begin to move slow when orbiting though, so I turn off transparancy (make the windows opaque in your video settings for slower cards, and make the sky one color, not a BMP or image).
Again:
AC will run on consumer cards without problems- get the middle range or better (check tomshardware or something similar to see what that is). Unless you're working in Dubai, you won't notice or see the money spent on a pro card, in my experience. It's better to get two in SLI or CrossFire than 1 super expensive card, for sure.
Hope that helps.