Dominion is brilliant, currently my favourite board game (though it's a cardgame techically). If you want to play with six, ideally you'll need the Intruige expansion as well as the base game. Both of these sets are £25-30 each. That said, at a push you can manage with just the base set.
I'm amazed that Settlers of Catan hasn't been mentioned yet. Also have a look at Carcassonne, both are excellent family strategy games that needn't take up a week of your life.
I also highly rate Smallworld, which is based on the (slightly more advanced) Vinci.
Stay away from Milton Bradley and Waddingtons IMO