8 nights in Prague or do I add Vienna or Budapest? Best $$ Trasportation?

Well, Prague to Vienna is a 5 hour drive away, then add another 3 hours to budapest from there. But with 8 days...it is definitely do-able For a 3 day rental of a car, you're looking at about 300EUR as a reference. I don't know the exact train fares, but you easily do the comparisons by checking Eurail for ticket prices and using Expedia/Travelocity for car rental prices. But of course with the car rental, you'll also have to factor gas and toll charges. My bet is it will be cheaper to take the train.

There's no shortest of things to do in either Vienna or Budapest. But instead of hitting 2 different countries, have you thought of touring smaller towns in Austria? Melk, Krems, Rust, Eisenstadt, etc....

Shelly,
From my expereience Vienna beats Budapest hands down. Praugue is a great City too. Beware of pick pockets who are very good at what they do. I learned first hand.

Shelly, I might add you could try the Danube River Cruise to reach either city. The Airport in Praugue has a booth for one of the lines. I can't recall for sure but maybe Avon. Very nice and one way to get there.

As far as economical means of transport, once you have your itinerary figured out you might want to check bus lines rather than trains. In some cases the bus might be less expensive (although train travel within the Czech Republic is relatively inexpensive). Two bus companies operating in the area that I know of (and have used) are Orangeways bus (orangeways.com) and Student Agency Bus (StudentAgencyBus.com).

I personally would not spend 8 nights in Prague, but it's also my least favorite city destination. I absolutely loved Vienna and Budapest, hard to determine the favored one between the two, but if budget is a concern then you may find Budapest to be a little easier on the wallet.

With 8 nights you could also easily spend all of your time within Czech Republic getting the flavor of the country outside Prague. For instance, we spent a few nights in Brno and loved it - not another tourist in sight and it's known as "little Prague" (they do, however, have a booming convention business so you'd want to plan around any biggies).

Lots of options, it really boils down to your travel style and preferences.

I would add Vienna, not Budapest. It is closer and I liked it more than Budapest. I do think a week in Prague is a lot. The trains are not expensive and you can travel in the evening, thus making the most of your daytime for touring. The bus is probably cheaper. The travel times are about the same.

I haven't been to Brno. It is between Prague and Vienna by train.

Budapest is a 7 hour train ride from Prague. Vienna about 4. Although I like Budapest better than Vienna, I would stick to what is more convenient to get to; Vienna.

Having been to both Prague and Cesky Krumlov. I would suggest you spend 2 days in Cesky Krumlov. You could even travel to Vienna and fly home from there.