Well when buying any type of pc the most important thing is your budget, how much are you looking to spend?
One site to give a look at is Alienwares,
Also cube24/7 have some good rigs too,
But which ever site you use make sure you can add or take off stuff when ordering, Both cube and alienwares sites let you do this.
Also consider whether you want to play a few older games too, this is important as you wont be able to play older games on a pc with Windows Vista on it.
Some manufacturers let you choose between XP and Vista.