I would also recommend taking a train. I find it a lot more comfortable to not have to drive 2-3 hours after flying for 8! The fastest connection from the Frankfurt Airport train station takes 3:19 hours and costs 44 Euros (or 66 Euros in 1st class). Other connections are also available. If you send me your arrival date and time, I'll send you a list of appropriate connections.
You might also try the German Rail web site, www.bahn.de. If you click on the tab marked "Int. Guests", you will find an English language search interface, and can even book a ticket online.