This question is probably best answered by one of the 'locals': what would be the best thing to do with buying tickets for public transport? Here in London for example we have 3-day travelcards for using buses and tubes. Is there some similar ticket in Berlin?
I was there a few months ago and remember being a bit confused by the ticket machines, which probably wasn't helped by the fact that I couldn't tell the train and tram machines apart at first. I ended up getting single trip tickets, which was fine for being there for only two days. Maybe someone local has a better idea though.
I suggest that you buy a day ticket (Tageskarte) in one of the vending machine you'll find in any stations in Berlin. It costs 6.10€ and is valid 24 hours in the AB zone of thez city after you have stamped it, which is about everywhere you'll want to go.
If you want to see how these tickets look here is one:
http://berlin.equipier.com/transports/tout-augmente-plus.php
Didier
And what about the Welcome Card? Is fot 48 or 72 hours (16 and 21€)
More info:
http://www.berlin-welcomecard.de/
yes, the welcome card is cool. :)