Just thinking about the old days here..
You want to have a wall of monitors where you can drag windows to any monitor and spot.. right? So that's extended monitors.
I don't know if you can drag windows across gpus though. I would be cool if you could.
I know Eyefinity needs active adapters but I'm not sure about extended screens on the same card.
Even though I use 3 screens, I use native cables (no adapters) so I can't be sure whether extended needs active adapters or it's just eyefinity.
you'll have to get informed about passive/active adapters if you're going to use DP to DVI or VGA.
Google amd 5870 active adapters
There used to be a member on Rage3D who built monitor walls as a job. You might want to search around there.