As for video cards, preferably go for as many outputs as you need on a single card, rather than multiple video cards. Just seems less driver issues and heat that way.
I am down to a single ATI2460 silent low-profile twin-GPU quad-mDP (mini DisplayPort) card now to drive 2 x 30" at 2650x1600, and 2 x 23.5" touchscreens at 1920x1080, with each GPU driving a 30" and a 23.5" to spread the heat, as every pixel displayed requires more power. AMD lists the card as suitable for driving video walls of monitors, and that is probably the class of card best suited for DAW work, rather than a video games card.
AMD OEM cards a bargain at Amazon US for overseas buyers
FYI, outside the US, AMD video cards can be purchased from Amazon US from most places in the world, which can mean substantial savings. My ATI2460s were just over a 1/3 the price that they cost in Australia, and that includes shipping. Unfortunately, Amazon doesn’t have the same arrangement with nVidia, and it is only for AMD OEM cards, not their partners’ ones.
I had originally been interested in nVidia NVS450-based cards, which had full-size DisplayPort (DP), rather than mDP, which I find rather under-engineered, with the cables putting a lot of stress on the non-locking connectors.
mDP cable rates
The other thing I found with mDP is that not all cables are equal. Most mDP cables will be designed for the likes of the Apple 2560x1440 and are rated for under 9GHz signals. My Dell 30" are 2560x1600 and would not work with some of the cables because they really need the 10+GHz ones. Once bitten, twice shy! The 10GHz ones have thicker cables, putting even more stress on the card’s connectors.
Fortunately, neither the originals nor the new cost a lot, but it was still money down the drain. After all the bloggers raging on about how cables did not make much difference to HDMI performance, I would be looking for frequency rating for cables in the future, especially for HMDI 2.0 ones, which because of a new encoding protocol, do not need anywhere like double the frequency of HDMI 1.4 to handle 4K at 60Hz, but it is still possibly above what the cheaper existing cables will handle.
The DisplayPort cables I have were supplied with the monitors and would probably handle the rates. They also look a lot more heavy duty than the mDP cables.
4K at 50/60Hz options
When we find suitable 4K TV/monitors, I will get a quad output card that can drive them. The problem is that even the second-generation 4K TVs are only using HDMI 2.0 rather than DisplayPort 1.2, which is the only 4K/60Hz option from current video cards. The upcoming Sony 49X850B/40X8500B looks to be a suitable size for DAW use.