Do you have the ADI Drivers installed along with the Radeon control panel? It may not do it through the OS, but it should do it through the Radeon Control Panel.
Also, some of their cards with 2 DVI and one HDMI will only use either DVI or HDMI of those 2 ports. All have to oh look up your GPU and find out if I can find the card that is on this particular Dell machine to see if that's the case here but that's what I suspect.
If you find someone on a dating website that knows as much as this about computers, they're a keeper. What site do you use?
Where are you changing the monitor configuration from? Windows, or the Radeon control panel?
What leads you to believe that this card allows you to use 6 monitors simultaneously? I double checked for you - it supports two, unless you use mini display port.