No display going to the monitor

Core 2 Duo
8gb DDR2 RAM
Windows Vista Home Premium
2 SLIed EVGA 9800GTXs
2 500gb hard drives
Some random DVD drive
1000W OCZ power supply
If you need to know anything else that I am forgetting then I can add it.

I built a computer for my friend a while ago and recently it stopped working. The power button on his computer wouldn't work. So, I tested the the power switch and it worked fine, I tested the power supply and it worked fine, and so I decided to replace the motherboard. After installing the new motherboard it seemed like the computer was starting up, I got optimistic, and was heartbroken when I then saw that when I turn on the monitor I got no response. Here is what I tried to get some response already:

-I took the SLI bridge off
-I took out one graphics card
-I then tried the graphics card in the other slot
-I tried the other graphics card in both slots
-I tried both DVI outs on both graphics cards in both positions, so that rules them out as the problem unless they are both the problem right?
-I took out the processor and re-set it to make sure I did it right, which I did.
-I double checked all the wiring
-I tested the monitor
-I tested the monitor cords, both power and DVI.

So, yeah... Before I go out and buy what I think is the problem(The processor), can you all tell me what you think? I don't want my friend and I to keep dropping money into this, especially if I am doing it wrong. It seems like it is the processor because everything turns on when the power is turned on, but nothing comes on screen.... which makes me think there is power, but no brain to start everything. I really need your help all because I'm just not sure anymore! Also, what made the computer break in the first place?

No, I didn't try different monitor cables. I am using the cable which I tested the computer with now on my computer, also it was formerly that computer's cable. There are zero problems with this cable.

I was saying the power button didn't work just to tell you what he told me. One day he came home, tried to turn on his computer and it wouldn't turn on, so he said, "the power button doesn't work anymore." My dad has some equipment that tested the wires and the micro switch and they both worked fine.

Oh, I forgot the SLI comment. Well, I think I addressed it in my last paragraph, the computer just wouldn't turn on.