I'm just presenting a wild guess now, but could it be that the PSU don't have enough power to run all the stuff that you put into the computer? That could explain why it's not working when you put in both of the graphics cards at the same time as all the memory.
I believe that the memory doesn't take too much power but it might be that you are at the edge already and this little extra power consumption makes the computer not to start.
As I said, it's just a wild guess but it is worth checking if you don't have a more powerful PSU.
Did you ever get this working? I have a very similar problem and am out of ideas...
A quick update.
I was NEVER able to resolve this issue.
I needed the 8GB of ram and also the three displays. So the only thing I could do was to replace the two existing video cards for one video card able to drive three displays.
I replaced the 8400GS and 9600GT cards for ans NVIDIA NVS 450.
@Phaser606, Your suggestion was interesting because the computer had indeed a lot of stuff running of a "regular" 450W power supply.
I bought a 750W Corsair "Professional" power supply to replace it and It did NOT make any difference what so ever.