Hambone... the trip you linked to is actually pretty decently priced, I'm impressed. I also like the fact that it's not a tour, it's just lodging and transportation... it leaves the rest to you to figure out, which is half the fun.
Sounds pretty good to me, if you really want to see those 3 cities in particular (though personally, I am dying to see Eastern Europe, but you gotta start somewhere).
My only concern is the date you chose... that's freaking high season, you can't get much higher... which means all those cities are going to be CRAWLING with other tourists underfoot. The weather will be good (most likely), but still... having so many other tourists can get really old. I recommend going during the shoulder season, for example in May (when there's a cheaper trip, anyway)--that way you have a *little* more space for yourselves (still lots of tourists, but significantly less than in high summer). But again, entirely up to you. Waiting in long lines to see the big sights, though, is not really that fun.