How much to know about a place?

The question of knowledge rears up again. How much do I want to know about a place, before I go there?

I've been skimming through the odd guidebook and article here and there, getting a bit more familiar with the 3 gajillion cities on my itinerary. There’s a lot to be said for just getting to a place and going on a whim and serendipitous spirit. No doubt there.
Since I only have a few days in each place, tops, I also want to have a pretty strong grounding of what each city is like. I don’t want to get to the train station, and have no idea which way is up. That just wastes time. Besides, I also know me. If I don’t have a decent info grounding, I have that wonderful luck of wandering about trying to find something, only to find out later that something abso-shaggin-brilliant was about 2 blocks over.
I want to be able to find the abso-shaggin-brilliant stuff, which should be made easier by knowing it ahead of time.
Though of course I’ll be talking to people, from locals to hostel managers to other travelers, so naturally I’ll get suggestions and directions that way. More knowledge is good though. Serendipity rocks, but I don’t believe in going at it ignorantly. If I don’t know Prague from Paris, then I’m just going to be wasting time, and I don’t feel that’s smart.
So… back to my reading…