Need HELP with XPS 420 and multiple video card issue and freezing with boot up.

Ok, here is my problem. I have a dell XPS 420 that came with the ati 2400HD video card.  I Like to use 4 monitors so what I did was take that video card out and installed 2 ati visiontek 9250 PCI video cards. This way it keeps the cost down and I can "normally"  run 4 monitors  ( 2 per card )   Each card occupies a PCI slot.  The problem is when I go to install the second card my computer freezes during boot up. When its in the Microsoft screen and the little blue dots are trailing across the bottom. that freezes up.  I have to kill the power and try again.  If I pull one of the cards out it boots right up without freezing. I cannot get a clear answer about this from anybody!  Does anyone know what is going on?  One person says PSU....  but I dont think that is the case...  I never lose power I Just freeze during boot up.  Help! Thanks.