I have seen lots of different posts on this issue, but I cannot figure out what to do. I hope we can collect the available information in one place, so that they can be useful for others.

Is it possible to have multiple (3+) monitors running on two different graphics cards?

I have

• 1x nVidia GeForce GTX 550 with 2 DVI ports and
• 1x Intel integrated graphics, with 2 DVI ports.

I understand that they would be running on different instances of X servers. Is that correct? Could someone point me in the right direction to start?

On Windows it's so simple, there is no additional thing to do other than going in display preferences and activating all 3+ monitors. They can even be laid out alternating one monitor from one graphics card with another monitor from the other card.