Not something daft like the VGA cards need powered seperately from the PSU? (usually a 4 pin connector)
Unlikely but you never know

Only thing I had similar recently was I had to update the motherboard BIOS on a customer's machine to run a new graphics card....!
The machine would POST and boot up but there was no display after the initial POST screen

This was a Packard Bell machine though.... what make is the motherboard?

 

This is driving me insane now! New PSU, CPU, Motherboard(Asrock) and still does squat. Have tried EVERYTHING. You can hear it boot up, but see shit on every monitor. The monitors work fine on my PC. Never had this before, must be the work of satan

 

Eil did you try what I said ?????? That is EXACTLY what mine was doing and I would thought the power supply would have been last on the list of things to change.