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?