Question New Video Card - "No Signal" to Monitor

[Yardstick]

I'm trying to upgrade the video card in an older Dell Inspiron 620 desktop that is running Windows 7. The computer originally came with an AMD Radeon HD 6450 card. We have now tried two new cards and neither puts a signal out to the monitor. We just get the "No Signal" message on the monitor. The cards we have tried are an MSI Radeon 550 and a VisionTek Radeon 7750. They are both PCIe x16 cards and both say they are compatible with Windows 7. What am I missing that is not compatible? Is there any way to get one of these cards to work in this computer?

 

[popatim]

Did either card allow the PC to boot up? (You heard the windows startup sound?)
Does the PC work when you put the old card in?

 

[Yardstick]

We can't tell if the computer is booting with the new cards. I'm troubleshooting this remotely so haven't been able to verify first hand. The PC works with the old HD 6450 card. The old card won't output 2k from the DVI or HDMI ports to a new 2k monitor so that's why we were trying to upgrade the card.

 

[Yardstick]

Quick update: It seemed like the power supply might be the limiting factor to getting the computer to run with one of the new cards. We installed a new 650W power supply and still have the same problem. Any ideas?