Also your latency plays a big part. If you use satellite internet your actual speed can be great, but by the time anything you do goes anywhere it will be about a second to a second and a half and is pretty useless for gaming.
Otherwise if you have DSL, cable, fiber, wireless internet etc the latency will be normal and your speed will work fine for most games. Just keep in mind that if another user on your network goes and uploads a bunch of stuff you are going to start lagging and having problems big time. DSL especially will tank both ways if all of the upload bandwidth is being used.