I have two nVidia cards in SLI powering 3 monitors.

Setup was pretty simple using the binary nVidia drivers on Kubuntu, 0 config file wrangling.

Now, having those three monitors in non-uniform geometries is something I wouldn't even consider tackling

[EDIT]

Kubuntu 12.04

All monitors in a single X Session and automatic application window sizing works as you would expect in KDE

(i.e. Maximising a window maximises to the current monitor only and being able to drag to right side of one monitor causes the window to use up half of the current monitor)

Can you get them all in the same X session? I've never tried SLI, I generally just upgrade to a beefier main card and use a second card to drive extra monitors. I have an HD6970 and an HD6450 on 4 monitors that work great under OS X (Hackintosh'd) and Windows and things go extremely sideways under Ubuntu.

I had 3 monitors / 2 Nvidia cards working fine in Kubuntu 11.04 and 12.04. All one X session. It was easy. But that was with uniform geometry. (I had 3 identical monitors.) Haven't tried non-uniform.

Ah well, Radeon with Crossfire under Linux is... yea, good luck. All I can say is nvidia in SLi works fine.

"fun"