Dell Dimension E520 Boot Issue

I am trying to help someone with their Dell Dimension E520.  The PC will not boot and there is no signal to the LCD Monitor.  When I turn the PC on the Diagnostics lights go through a sequence and eventually stop with lights 1, 2 and 3 being on.  Immediately after the Diagnostic lights stop with 1, 2 and 3 being on, I hear two beeps of the same length.  I switched out the Power Supply with different Power Supply, but there was absolutely not change, so I switched back to the original Power Supply.  The CPU heat sink/case fan is running when the PC is on.  I have already re-seated the video card and checked to make sure that all connectors are properly connected.

Where do I go from here?

Thanks in Advance.