It sounds like there may be an issue with the compatibility of the new graphics card and your Dell Inspiron 3800 series. It could be that the power supply you purchased is not providing enough power to the graphics card, or there may be a driver issue.
First, make sure that your power supply meets the minimum requirements for your graphics card. You can usually find this information on the manufacturer's website or on the packaging of the graphics card.
Next, try updating your graphics card drivers. You can do this by going to the manufacturer's website and downloading the latest drivers for your specific graphics card.
If neither of these solutions work, it could be that your Dell Inspiron 3800 series simply isn't compatible with the new graphics card. In this case, you may need to consider upgrading your entire system to a newer model that can handle the graphics card.