You do not need a dual link DVI for 1680x1050, even 1920x1200 does not need it. You only need dual link DVI for the Dell 30" monster at 2560x1600.
Two 24" panels at 1920x1200 will give you 4.6 million pixels, one 30" at 2560x1600 will give you 4.096 million pixels, so they're very close...