Well there are quite a lot of mcdonalds n stuff around the Hauptbahnhof and Reichstag. You shouldn't eat in a Restaurant near the Reichstag since its kind of an expensive area (beside the Dunkin Donuts stuff)
Depends on what you would consider as a "decent meal"
In the old city centre of potsdam there are quite a lot of typical German Restaurants.
If you walk down the Kurfürstendamm there are some Sportsbars and some Restaurants they are a bit expensive
like.. dunno... 2? > the typical meal prices...
Prenzlau is more a bar area as well as Kreuzberg but there are quite a lot of restaurants all around the places you mentioned, just go in some random place with Kartoffeln on the Menu (Potatoes n Stuff) and you will find a lot of Restaurants. On the Potsdamer Platz, near the Reichstag (5 min walk), there is a nice steak house @the Casino on Potsdamer Platz or some Restaurants in the Arkaden (1 min from the Steak House) or a Sushi Bar next to the Cinemaxx. Potsdamer Platz is the Place where you will find a loot of places with decent food ^^...
well however the next time im going out for dinner @potsdamer platz i'll have a look @ the prices.