[Solved] New VGA - No video + Motherboard beeps (1L-3S)

Hi, I need your opinions on this strange case. First my specs:
- CPU: Intel Dual Core E2160 @ 1.8 GHz
- MB: Asus P5G41-MLX (Intel® G41 + ICH7 chipset)
- RAM: 3 GB DDR2 800 (2+1)
- PSU: Topower M5 Silent 650w
- HDD: Samsung SATA 160 GB

So, everything was working perfect, had a XFX 8800GS videocard, no issues whatsoever. But this year I want to update my rig, buying parts little by little, so I started by getting the bew videocard first, a Sapphire HD 6850. And that's when all hell broke loose...

When I arrived home from the store, I replaced the 8800GS with the HD 6850, plug the PCIE power cable and proceed to turn on the rig... NO VIDEO!! And on top of that, beeps from the MB: 1 long beep and 3 short ones. Obviously I re-checked the connections, but nothing. Removed the VGA, clear the CMOS, turn on the PC without the card (using the integrated video), and got video again! Plugged back in the VGA, disabled the integrated video, reset and... no video and the beeps.

Thinking that maybe it was a MB issue, I used another MB (ECS G31T-M), but again, no video (the beeps didn't appear, though). Then I looked on the net for the beeps and seems that, on ASUS MB, it's a signal for "Bad Video Memory or VGA not detected". I plugged in the old 8800GS again and it worked flawlessly on both MBs. "It's the videocard then" I thought, and rushed back to the store. But when they tested, IT WORKED!! They used a Core i7 type of MB to test it.

To keep the story short, this is a list of tests I did when I returned home, after searching on the net and finding other similar cases of ASUS MB users (in most cases, they couldn't solve the issue either):
- Replaced the MB's battery (of both MBs)
- Dissassembled and assembled everything
- Turned on the system without the CMOS battery
- Upping the voltage of the RAM (from 1.8 to 1.9)
- Tested each stick of RAM individually, on both slots
- Cleared the CMOSes several times

NOTHING WORKED. I'm desperate, having spent the money and not being able to solve the problem. Any ideas? I'm thinking that maybe the HD 6850 isn't compatible with PCI-E 1.1 MBs, and that's why it worked on the test rig in the store. Will I need to replace the MB? Thanks in advance!

Another thing I forgot, after the beeps (with the HD 6850), the system kepps loading the OS normally (heard the logon sound of windows 7), but the monitor keeps black (in energy save mode). The monitor is connected via DVI-VGA adapter.

Thanks for the reply!
Aside from the PCI-E part, I think my PC covers all the requirements. The PCI-E x16 slot is version 1.1 on both MBs. I only use one LCD display

Best answer selected by edgardo-xp.