I have a PC with 3 multihead graphics devices, but I'm unable to have more than 3 monitors at once.

I'm using a P8H77-V motherboard. It has built-in Intel HD graphics (display port, DVI, HDMI). This seems to work (although I don't really plan on using it).

I've got an AMD FirePro V4900 plugged into the first PCI-E slot, and I'm currently using all 3 ports on it with no problem.

In the second slot I've got an MSI Radeon 6850 with two DVI and two display ports. Any 3 ports on it will work simultaneously.

The problem is if I plug a monitor in to the 6850 while I already have 3 monitors on the V4900 (using an active display port adaptor on the 6850) it doesn't work. As soon as I unplug one of the 3 monitors on the V4900 the one on the 6850 starts working.

The same thing happens if I try to move one of the V4900 monitors onto the onboard Intel ports. All the ports work fine, until I have 4 monitors in total.

Now, I know that some cards don't support using all their ports (for example, I believe my 6850 wouldn't accept 4 simultaneous outputs), but surely there shouldn't be a limitation across multiple cards?

I don't believe that this is a software issue. I'm using Windows 7 at the moment, but an initial investigation seems to show the same with Debian, though there may be driver issues at play there (I want to transition to Debian with the V4900, with a KVM virtualised Windows machine using a single monitor on the 6850, but I've shelved that until I can make all 4 heads work in any operating system!)

Is there some kind of hardware limitation at play here?