I think there needs to be some stability with 4k monitors and refresh rates, there was another thread I asked a question in about this.
I have a 7970 from MacVidCards and while it'll support the resolution, depending on the monitor and card, may only support the resolution at 30Hz.
I was/am in the market for 2 27" ACDs but the price of those Dells coming out in a few weeks really changes things for me, especially if I won't have a problem running them...
I don't know if it's going to be more of jus what specs a card or a monitor says it can do as opposed to some trial and error until 4k becomes more of a standard.