i know that the Mac elite has a list that shows what will work in what system

http://themacelite.wikidot.com/compatibility

it looks like the 6200 is the latest card that will work in a 2x slot for nvidia cards. and the 9200, 9500/9700/9800 are the latest cards that will work in a 2x slot for ATI cards.

i would try to find something that supports both QE and CI.