|"if I add an AGP card will it override the controller on my motherboard?"|
You will most likely have to alter some BIOS settings to override the onboard graphics.
"Will I have to replace power supply if I ad Video Card?"
Maybe, maybe not. It would depend on the specs of your current PSU & which video card you choose. The Gateway component list says that you have a 250W PSU but two different units are listed...one from Delta & the other from FSP. Unfortunately, the amperage specs aren't given. My guess is the FSP unit would be the better of the two.
Here's your motherboard sepecs: