Went in March with my sister. Were there for 6 nights and that was plenty of time to do pretty much everything (all the touristy bits) in Rome.
Wednesday - Arrived Rome in the evening, found hotel and went out for a meal.
Thursday - Woke late (lunch time) and walked down to Colosseum via St Maria Maggiore church and St Pietro in Vincoli churches. Had a guided tour of Colosseum followed by tour of the Palatine. Had a quick bite to eat from a snack van (pizza sandwich omnom) then met up with our tourguide again for an evening "Secrets of Rome" tour, which included Michelangelo's fake and real tombs, a small church (seats 6 people) which is a replica of St Peters, Colonna M Aurelio, Pantheon, Trevi Fountain and Spanish Steps.
Friday - woke early to get Metro to Vatican for a tour. 2.5hrs of special(ish) access from the same tourguide we'd had the previous day. Lunch then round to St Peter's Square. Walk down to Caste S Angelo and walk back through Rome to our hotel via Pantheon, Trevi and Spanish Steps (this time in daylight).
Saturday - walked down to Colosseum again via San Pietro, this time popping inside to see Michelangelo's "Horned" Moses. Saw fancy arch next to Colosseum (can't recall name) then round to Circo Massimo and Mouth of Truth, then back to hotel.
Sunday - Metro to Flaminio and a nice walk around Villa Borghese park, visiting the replica of the Globe theatre, metro back to Spagna for evening meal then to a local bar for drinks.
I spent Monday in bed with a rotten cold unfortunately, then flight back on Tuesday. As we'd crammed in all the sights we didn't really miss out by not having a day out on the Monday.
To conclude, Rome is awesome, DO EVERYTHING.
Edit: Oh yeah, March is their off-season and everything was still uncomfortably busy, especially (obviously) the Sistine. Would hate to be there in the summer at peak time. :s