PokerTracker

Does the hud show only the active database stats?

And what is a smart way to manage ur database to not get it too slow and too big?
Like a database for every site or a saparate database for observed hands? But will the hud then only show the observed hands stats if u set that database to the active one?

You are able to choose which databases to retrieve stats from for use on the HUD.
On the HUD config window there is a tab "HUD Options", on here you can specify which databases to use.
(Note that currently there can be problems with selecting multiple databases but only seeing stats from one, but this will be fixed.)

As for database management, PT3 can handle pretty big databases, but if you're going to do a lot of datamining, the recommendation is to use a separate DB for observed hands and rotate this once a month or whatever so that it doesn't get too big.