Even the lowest card will do the Sims nicely.
Since it's a new machine the choice will be on the PCIe list.
As to the power supply, compare the card's requirements with what PSU you find in your machine. More at http://support.dell.com/support/edocs/systems/dim8400/SM/index.htm
I have a Dell Dimension 8400. I want to upgrade the video card. Does anyone out there know what I need to look for? What the best card would be for my use. I am a light gamer playing games like the sims 2 and sim city societies.
Should I plan on installing an upgraded power supply unit in association with the video card?