Karen - Sardinia is certainly beautiful and well worth a visit but as mentioned in previous postings it can take much of your time.
If you really want to go, take a look at corsica ferries . There are about four or five ferry services from the coast of Italy to Sardinia, but these folks are probably the least expensive. The trip from Civitavecchia to Sardinia can take about 4.5 - 7 hours depending on what and when you book.
To save time, book an overnight trip FROM Civitavecchia. They usually leave at 10 PM. You can book a cabin and sleep! The ferry arrives at 7 AM and you're ready to go! On the way back, take the 8 AM ferry, arriving back in Civitavecchia at 1 PM... catch the train and you're in Rome by 3:30 PM. We've done different routes but that seemed to work best.
On the ferries you can book a cabin, a pullman seat (like a first class US domestic airline seat) or NOTHING AT ALL (seating selection). On the return trip, I would book NOTHING - save your $$$ and lay out on the deck and catch rays!
Depending on your time of year accommodations in Sardinia can be scarce, so book ahead! There are MANY places to stay. Of course, as you're using the ferry, you can take a car with you - or rent one on the island. We went by motorino which was fun zipping around the hairpin turns on an island covered in steep hills, beautiful coves, and great beaches. It can be an incredibly relaxing experience!
But again, it is a chunk of time... and if you could change it, I would do Venice-Florence-Sardinia-Rome in that order... although I'm not sure you can do that in 10 days... but that order brings you Civitavecchia heading south to Rome. Might eliminate Florence (OUCH!). Hope that helps.
Ciao,
Ron