Best (time and expense wise) is probably (1) Metro to Roma Termini and the (2) Train to Civitavecchia. For info on the later, see the top post in this forum.
Radiance of the Seas - Nov 2006 Legend of the Seas - Jun 2007 Costa Fortuna - Mar 2008 Legend of the Seas - Oct 2008
Celebrity Constellation - July 2009 MSC Orchestra - Jan 2009