Installation problem: no video signal at all

I'm building a pc, but when turning it on there's no video signal at all. I tried every combination you could think of. Even tried another monitor.

Here's the specs:
Asus V4-m3a3200 barbone
AMD Phenom XII 955 black edition (AM3 on an AM2 mobo, but shouldn't be the problem according to Asus)
2x OCZ PC2 6400 4-4-4 2GB RAM (I tried every combination of the RAM strips, also with only 1 RAM strip)
500 GB Western Digital SATA hard disk (tried with and without it, even tried an old IDE drive but still no video signal)
LG Flatron 21,5" screen (tried both DVI and DSUB connections, also tried another monitor)
Asus EAH4850 GPU (works in another PC) (also tried everything on the onboard chip of the motherboard)

I even tried disconnecting EVERYTHING. I only had the CPU and 1 RAM strip in the computer. And the monitor of course (on the onboard video chip). Still no screen. I guess that in that situation you should see the BIOS loading and then complaining about no storage device found or something...

The only behaviour that might indicate to something is that when turning on, all the fans start running. But after a second, the chassis fan (rear exhaust) stops. This could be wrong, but I don't think so because it could be the BIOS stops it when the temperature is not too high.

Any ideas on how to make it work? It's just no video signal coming out of the 2 gpu's I have. Where does it go wrong?