Did this last year, there is sooo much to see, you'll struggle to fit it in in a few days or you'll miss stuff.
Highlights for me were
Pegasus Bridge Visitors museum
American War Cemetary - well not so much a highlight it was quite sobering, but something I think everybody should see.
I did a write up with pics here if you havent already seen it
http://forums.sv650.org/showthread.php?t=118005
If I was going again. I wouldnt get the night ferry. I'd take the Fast ferry and leave myself the whole day to travel where I was going to and vice versa when coming back.
I reckon you want a good few days, we struggled to fit everything in in the time we had.
Theres a guy on the gixerjunkies forum who does guided tours, he's a biker and his tours are setup for bikers, he has a place to stay as well. So might be worth contacting him.
Heres his website, I've nattered to him a bit on the junkies forum, seems like a decent guy and very knowledgable about his subject matter...
http://www.freewebs.com/gixajak/