It'll look like crap. 30" displays are supposed to run at 2560x1600. The iMac cannot power that. Save your money and get a 23/24" 1920x1200 display that the iMac can power.
I was told that even though the 27 is bigger in size, it has the same amount of pixels as the 24. Is that correct? If so, would it be the 24 or 27 that you'd recommend?