Just flew out of FCO to the US yesterday; I urge anyone flying out to allow at least 2 1/2 to 3 hours before flight time to get there. First of all, from the main airport to T5(virtually all US carriers) two shuttle bus' arrived at the stop(outside T3) for several 100 travelers and their bags; the scrum of people fighting to get on the bus was not fun. Then, check in twice at T5, go through security, bus to the remote terminal. I don't know if you can taxi directly to T5, but if you can, you will probably cut at least 30 minutes out of the ordeal. Finally, be prepared for a complete carry-on search before boarding along with a pat-down for every other boarder in the tube to the plane.
When I was in Rome last September my taxi driver took me directly to T5. I guess I didn't know what a blessing a smart taxi driver can be!
The big issue is you need to understand your flight. IF you are flying directly from Rome to the US then you need to go to T-5, the international terminal. If, however, you are connecting in Europe for your flight home via London or Frankfurt, which is common, then it is the regular domestic terminal. Last year a family member was returning to the US via London and made the mistake of thinking she was on an international flight. By the time she had fought her way back from T-5, she missed the 45 min cut off time for checkin and was forced to spend the next 24 hrs in the airport.