If your Graphics Card (GPU) can output a signal at 1080p, then you can output this to your TV. If your graphics card is only capable of outputting a signal at 720p then you will be unable to send a 1080p signal to your TV.
However saying that, some TV's will 'upscale' a 720p signal to 1080p but I'm sceptical that this has any meaningful benifit...