I'm trying to purchase 19x8.5 front and 19x9.5 wheels for my 8.
I want as thick lips as possible for both front and rear.
Can someone explain what offset means and what kind of offset I should go for
if I want thick lips?
Thanks.

 

Offset in a nutshell is how far in your rims are. The standard offset of an RX8 rim is 50 mm. So therefore if you chose a rim that has a 35 mm offset the rims would be 15 mm further out (towards the outside of the car). My 19" were 38 mm offsets upfront & 35 mm offsets in the rear.

 

A good reference:

http://www.tirerack.com/wheels/tech/offset.jsp