The GeForce 6800 DDL will support two 30" displays. So you could connect one 30" and one 23". Or you could buy the card simply for its very high OpenGL performance and use any monitor(s) you choose (though this would be pretty expensive just for games...).
All other Apple-supplied cards can drive two 23" displays; they do not however support even a single 30" at all.
Oh, and if you do decide to do a dual display 30" plus anything, pictures are a must!