I'm glad I came across this thread. I have a couple of different systems one of which uses 3x ASUS 27" monitors and two other systems that uses one AURIA EQ276W 27" WQHD IPS 2560x1440 monitor each.
I was thinking of upgrading my Wife's system which uses one of the AURIA EQ276W 27" Monitors with a Samsung 28" Ultra High Definition 4K LED Monitor (U28D590D) which has just now (today) actually reached its MSRP of ~$699.99 USD (before it was inflated to around ~$859 USD on Amazon, Newegg and elsewhere).
This would help free up her 27" monitor and I could basically just add the other 27" monitor as well (if I use a KVM) to the system using the three ASUS monitors. That would be five monitors on one system and I could always add a sixth monitor later. My problem was that I didn't really know how to add more monitors on my current GTX670 and this thread helped with that by suggesting the addition of a second card.
Edit:
I think I would probably want to go with dual AMD R9 280 / 280X video cards for the 5 to 6 monitor setup (assuming OS X Mavericks will support this) and move the GTX670 to the single monitor Samsung 28" U28D590D 4K monitor system.