This is a great screensaver to put you in the mood.
http://www.italyguides.it/us/roma/rome_italy_travel.htm
We were there in January - fantastic. It is very easy to see on foot as famous places are fairly close to each other. Metro tickets cost 1 euro and lasted for up to about 1.5 hours. An all day ticket costs 4.5 euros (I think), but when we travelled somewhere we found the 1 euro metro tickets the cheapest. We took the metro to St Peter's on three occasions. Did the whole thing and agree with AdamW you must go up to the dome - spectacular. We spent so long we were too late to go in the Vatican Museum (where the Sistene Chapel is) as it closes at 3.30pm. So we went back another day to find it was a public holiday - so closed again, but we thought there were masses of people going towards the Museum and St Peter's and found ourselves in St Peter's Square while the Pope was addressing the people - quite spectacular although we didn't understand a word and are not Catholic, but still felt a fantastic moment. Needless to say we went back on a third day and finally went into the Museum and the Sistene Chapel. I had been in there before and it was very crowded and couldn't wait to get through to the other end - sad - but when there were so many going through you couldn't really appreciate the beauty in the same way as taking your time. From St Peter's we walked across the bridge and through to Spanish Steps and Via Veneto (designer fashion galore). On another day took the metro to the Coliseum - stops outside. After that walk across to the Forum and ruins - that takes some time and energy - but well worth it. We walked down to the Victor Emmanuelle Statue - magnificent and then down to the Pantheon and Piazza Navona and finally the Trevi Fountain to throw a coin (means you will always return- and I did!). The beauty of Rome is that you can turn any corner and find something magical and fountains galore. It really is a wonderful place -I can't wait to go back again.
That aside the food is fabulous and we found eating out to be fairly inexpensive. You can get caught out with the odd drink next to one of the big attractions - but you expect that to a certain degree. I hope you enjoy it as much as we did. When I first went years ago it was August and far too humid - but in January - it was brilliant sunshine, yet cool enough to enjoy walking. It does get dark earlier, but Rome by night is even more spectacular.
