I think that sounds like a great plan. Warsaw is a VERY different city from Krakow, and most of the touristy things can be done in a day. The rest of the city is a mix of modern/communist and probably not of much interest to tourists. The old town is very nicely restored after the city was almost entirely destroyed in WWII. There's a lot of great and fun new restaurants, bars, clubs, etc...and the city is starting to take on a little bit of London or NYC feel...which could be interesting depending upon what you like.

Krakow on the other hand was never destroyed and has an amazing wealth of architecture and things to do. You could easily spend 2 days wandering around the city centre. There are innumerable churches, art galleries, museums, and gorgeous streets to hang around. There's also a great night life in the city square and in the jewish quarter on weekends. Wawel Castle in Krakow is also gorgeous...but plan well b/c their hours are a a little weird.

Auschwitz can be done in 1/2 to 3/4 of a day....returning to Krakow for dinner etc. You can either do it via organized tour (travel there by motorcoach and spend a few hours there--its about an hour or so away). Or you can hop on the train, and then take a cab from the train station and do things at your own pace. (Oswiecim is small...so it would be short and cheap cab ride). Make sure you see both camps when you are there--Auschwitz & Birkenau as they are very different and had very different effects on me. You should also check out the Wieliczka salt mine...which is very impressive (underground carvings/underground cathedral/underground lakes). This can be done in 3-5 hours.

Let me know if you have any more questions. I may be a little biased as I am originally from Krakow...but I think most people that have been to both will confirm my account. PM me if you have any specific questions.

-W