From Frankfurt to Roma, How?

The best way to get from Frankfurt to Rome could be to fly with Lufthansa (www.lufthansa.com) or Alitalia (www.alitalia.com) from Frankfurt Main Airport to Rome... While Ryanair (www.ryanair.com) also do flights from Frankfurt Hahn Airport to Rome...
Other cities that I would suggest you to visit around Europe could be Pisa for the Pisa Tower... Other cities would be Prague (Czech Rep) where they have manage to keep a large number of European historical building style and Vienna (Austria) or Venice (Italy) are not bad choice as well... But of course, this depence on what you are actually interested...
Within Europe, you can also look at EasyJet (www.easyjet.com), Wizzair (www.wizzair.com), SkyEurope (www.skyeurope.com) which all provide pretty low fare tickets around Europe...

It will be too far and expensive if you go to Roma on train from Frankfurt. I suggest you can go to Roma by air. Ryanair has a daily flight from Frankfurt-Hahn to Roma Ciampino. A single ticket only 15 euro. http://www.bookryanair.com/skylights/cgi-bin/skylights.cgi?language=EN
However, the flight will depart at 6:10am. But even you have to take taxi to airport, it's still cheaper than going there on train.
For other European cities, it depends how long is your holiday. If you have only 2 - 3 days, then visiting Roma will be enough. If you have around 5 - 6 days, you can go to Florence and Venice as well.