[Build Help] Having trouble installing graphics card.

I got a stock computer a year ago with a really good processor but a really bad graphics card. It came with a Radeon HD 4200 and I bought a Radeon HD 6670. I opened the computer up and there wasn't a graphics card in the computer. I only have 1 slot in my computer for a graphics card and there isn't one in there but my computer uses a 4200? I'm very confused. I also tried to plug in the graphics card without removing the card I couldn't find and my computer still says I'm using a 4200. Very lost here.