optiplex 755 usff video card upgrade

I have an Optiplex 755 usff that I got from my school and it was in a perfect condition and it came with 2 GB of RAM for max RAM and it came with the disc drive so that I could read and write DVDs and CDs. How can I upgrade the graphics in it because when I launch a game it looks all weird and I can't see almost everything in it (the games I'm testing are tf2 and portal)?