I would say as a rule thumb, White should not take the d5 pawn without a plan. There are however variations where he does, specifically the exchange variation where White wants to play a minority attack on the queenside.
When should white take the d5 pawn in Queen's gambit declined lines?
Get the old Sadler book on the QGD, and study it carefully. One of the very best opening books ever written, and quite suitable for new players.
http://www.amazon.com/Queens-Gambit-Declined-Matthew-Sadler/dp/1857442563
Can someone give me some general rules for the QGD? Mainly when should white take the d5 pawn. Thanks in advance.