Try another DVI port.

If Windows gets the main monitor port switched and the desktop isn't extended, the monitor might be blank.

try swapping the video cards around.

I already switched the gpu's around and tried different connectors for my monitor; on both video cards. Still nothing :(...

Can you hear your computer login to windows?

Maybe try with just one installed in top/main slot.

Try Safe Mode.

Remove all 590's and drop in an old video card and try again for safe mode.

Like Sajin says ... it would be helpfull to know if you are just getting "no video" or is the PC not even booting up.

Did you ever get that UPS Battery unit? You might want to do that before installing any more new computers. Dells aren't perfect, but 9 machines points to possible external problems at the installation location.

sounds like a dead motherboard.

Computer is running, fans are running, but no picture and no sound of the os booting up.

Yeah, they're going to have a tech replace it. :/

dang. =\

this will be your 9th replacement? thats just sad.