ATM Barlow Question

[Matt Devine <mdevine@nuera.com>]

For my 6" F/8, I have considered using a 25 and a 16mm eyepiece.  I had planned
on buying either a Meade 2X shorty or Celestron Ultima 2X Barlow to obtain two
additional magnifications. 
I was told that Barlows in general add to a dimmer image, (light traveling
through more glass)and would probably be better off just using eyepieces of the
desired F/L.  Is this the case, or only the case with inexpensive Barlows?
Also is 160X pretty good for planetary viewing, or should I push it higher.  I
don't think I should go higher  than 200X with a scope this size even though in
theory 300X is the max useful power.  Thanks again...