GT3 sold well, but then there was no competition (no Forza).
PS3 has a smaller install base and competition.
Halo sells regardless of it being a sequel, prequel, add on or spin off. Similar to how Mario sells well regardless if he is cart racing, crossing galaxies, playing sports or traveling around the world.
Halo is possibly one of the biggest reasons why there still is an xbox gaming division to this very day. If it were not for halo, would the 360 be where it is now?
GT being as brilliant as it is shares the burden of responsibility with a host of other games and not the core reason why Sony is still around today.
I am not saying GT5 wont do well, I am saying it wont do as well as Halo Reach. The gamers and their fans are totally different. Halo is more of a casual game compared to GT5 being a driving simulator.