I've been to Auschwitz. IIRC, it was about a 40 min. bus ride from Krakow. Krakow is totally awesome, sort of like a small version of Prague. It'd be out of your way, but Malbork, formerly known as Marienburg when part of Germany, is near Gdansk and has the largest castle in the world. Warsaw is probably halfway between Malbork and Krakow. IMHO, the Warsaw Ghetto Museum was more interesting than Auschwitz.
Be sure to check this site out.
Third Reich in Ruins
If you go to Prague, be sure to go see Lidice near Kladno. It's the village that was wiped off the map as reprisal for Heydrich's assassination.
I'll be in Prague (again) in a few weeks and plan to go see where Heydrich's car was hit.