I’m shocked nobody mentioned to check the voltage on the rails.
If your current PSU supplies enough watts, it doesn’t necessarily mean that your video card will go with it. Check if it has the necessary 6pin/8pin connector(s), and if it has enough volts on the rails.
If it doesn’t, you can forget about buying a new GFX card. You would need to upgrade your entire PC (in most cases, the power supply and MB of Dell PCs are proprietary, and upgradability is limited at best.
Really be careful when it comes to upgrading OEM PCs. Once you spend the money, its often too late to go back.