I can help you out with Berlin, Vienna & Venice.
Berlin you would probably want to spend 3 days in.
Its a huge city and all the sites are spread randomly all over the city. The good thing about Berlin is its transit system, which is really well done and fairly easy to figure out. Most the big locations (Check Point Charlie, the Memorial Church, the Zoo, the Berlin Wall) all have transit stations near by.
Vienna you could probably cover in 2 days. Alot of the buildings there (parlement & churches) are really well lit at night.
I was in Venice for 2 days but wanted to stay longer. Although some people will say 1 day is enough...the canals all start to look the same after a while
I really liked it there, but it was a bit pricey. The only hostel I could find was on a different island, I was travelling with a couple other peopel so we managed to talk down the owner of a hotel to a fairly decent price. I think we paid about $25 euro a night, which is dirt cheap for Venice.
& since you seem to want to make your way from Berlin to Vienna. You should look into stopping by Munich & Salzburg.