It might be worth you look at parking at one Romes airports for 3-4 days and travelling in by bus. I am not sure where you would park in Rome or the cost as I am pretty sure its at a permiumm.

Sarah

Old post i know but just to say, parking and driving in rome is a nightmare dont even think about it
You arnt allowed to drive in ther centre with out a permit anyway (as far as i know) and driving is chaotic. Parking is not easy.
To put it in perspective my close friend who has lived there all his life has problems parking when we go out in the evening for a meal, and he drives in rome for a living.