Its a dump. Go to Florence instead.
Aside from that though, I was there for just under a week a few years ago and it would be really easy to fill the whole week with museums and cathedrals and such, but you should really prioritize what you want to see so you're not running around like a lunatic.
The Vatican and St. Peter's are a must though. If you're going to skip anything I'd skip the Coliseum, definately not as cool as you think it might be, not to mention you might run into the Senator's Gladiator Guy, he moonlights there.