First of all, take a moment to think hard about whether you really want to deal with the hassle of dual video vards. A few things to consider, based on a coddinghorror blog post about it.
- Power consumption: two GPUs will keep your PSU working very hard, and possibly cause it to overheat. Your PSU may not even have enough power to run two cards at once
- Instability: running two GPUs at once is at best a stable as running a single GPU. In reality, dual-GPU setups introduce a lot of issues. It's a very inexact science.
- No overclocking! The GPUs have to run at the exact same configuration, and should be left in their default config. The CPU should also not be overclocked.
Instead of using 2 cards to power 3 monitors, you should be looking at a single card which can support 4 displays. It'll be expensive, but less expensive than two cards + a new power supply. AMD/ATI makes several high-end cards which support up to 6 displays. Remember that you can only use up to 2 DVI, HDMI, and/or VGA outputs on a single card; anything above that must be DisplayPort. Get a card with at least 2 DP and at least 2 DVI (or with 4 DP), and you'll be able to run 4 monitors from 1 card with less power consumption, more stably.
This was already answered but I want to add some more details:
If you are going for gaming performance:
One fast card is better than to separate cards. Two cards tied in crossfire or SLI have lots of additional contraints, such as similar or identical GPU's, only the graphics out output from one single card active, etc etc.
Given the last this is not what the OP is asking. Two cards in crossfire would only serve 2 displays. Not 4.
For just more displays and not more performance:
Two cards will work just fine. It is easiest (from a driver point of view) to have two cards of the same brand, but you should be able to mix and match any pair of cards.
The card you listed are workstation graphics cards. Both AMD and Nvidia have these. They do not perform any better than regular consumer cards. They are often slower. They are a lot more expensive.
They do have advantages in some case though: E.g. better tested drivers. Build for max stability. Often much better performance when used in calculations on on the GPU (not the rendering calculation, but OpenCL and friends).
So in the OP's case:
- If you need warranty and have a big purse: Yes, buy a pair of these.
- Alternatively buy a cheap consumer card to connect the third and fourth display to.
- Or buy a single card with support for 4 or more displays (I know both AMD and Matrox have cards like this. Other brands possible to though I did not check this).
One more note: Graphics cards can be power hungry. If you do not need performance but you are limited by the amount of (+12v) power from the power suply, try to get a lower end card. For office work these still work fine.