New for 2005 GTO is

400hp/400ft-lb LS2 engine
Hood scoops standard
Dual sided single pipe exit exhaust
Upgraded (larger) breaks with red calipers with white GTO painted on them.
2 new colors
That is the only changes other than a $3000 appearance package that is a dealer installed option. And 18" wheels availible later next year.

Goto pontiac.com and click on GTO they have updated the material to reflect the 2005.