Hey,
Here are some pictures that gamereactor (Sweden) compared between Forza 2 and 3..
http://www.gamereactor.se/artiklar/19586/Grafikduell%3A+Forza+2+vs+Forza+3/
(I fail on putting up the screens so just go here and press next etc: http://www.gamereactor.se/bild/?textid=19586&id=193645 )