Agreed about the "choose your countries" then you can look at climate. I would suggest September or October. Airfares are insane in the summer.
Do you want to see art? Cathedrals? Nature? Historical sites?
Go to the site of a tour company, like Globus, and request some of their free brochures. This will give you a great idea of what sort of itinerary to plan, how much you can accomplish in a certain time, and what sights there are to see.
We are in the early planning stages of a 2010 trip and although we're planning on going on our own, the brochures have been lots of fun to pour over. We did go with Globus before and enjoyed it very much. We were able to cover a lot more territory than we would on own own, but we had a different agenda for that trip. DH had never been to Europe and we wanted to see everything possible in the time we had. A tour will drive you up to the major attractions, let you out, and then collect you in a certain amount of time. They handle all your luggage. All you have to do is show up

However, you don't get to experience as much of the local culture, nor do you have the option to leave early if you're bored or stay longer if you're interested.
You'll have to weigh out which is best for you
Our Globus trip covered 17 countries and of course we just hit the high points of a few major cities. Our upcoming trip will be Germany, Austria, and Italy, and we want to go at a slower pace. Also, we're comfortable using European public transportation. A tour is prepaid, so even if the dollar falls, your price doesn't go up (usually...if you're paid in full); on your own, you're hostage to the exchange rate a little more.