Almost all fans are 120mm now, bigger, slower RPM = more efficient.
Do it mate, nothing wrong with having an exhaust fan on the back of the PC, in fact i'd say if i was you i'd definitely have one there. Especially with a 'gaming PC' with today's hardware in, they can get quite hot.
You can see pics of my rig now with the fans on
here, it would go into meltdown without an exhaust.
I forget how this turned out BTW, Did you get the PC back with new card and OC or is it still running @ Stock speeds?
Also, Post up a pic! I wanna see their work.