Two integrated DVI display outputs
Primary supports 18-, 24-, and 30-bit digital displays at all resolutions up to 1920x1200 (single-link DVI) or 2560x1600 (dual-link DVI)2
Two integrated dual-link DVI display outputs
Each supports 18-, 24-, and 30-bit digital displays at all resolutions up to 1920x1200 (single-link DVI) or 2560x1600 (dual-link DVI)2
Notice that both the display outputs of the the 4850 are specified to be able to handle resolutions up to 2560 x 1600, whereas only the primary display output of the 4670 is said to be able to handle that. What the secondary is capable of is beyond me, which is why the 4850 seems to me to be the safest option.
so i can use the imac as a back up machine to my Mac Pro, without having to use the glassy display, which maybe more of a distraction and a waste of energy having it on. the matte on the 30 ACD and the glassy 27 display would be a pain
Someone else should confirm this, but I think if you turn the brightness down the the minimum, it turns the screen off. This might just be for laptops, but maybe there's a hack to do it?