While I expect Reach to become the biggest 360 game ever as this will be its peak title, should probably easily pass H3 and MW2.
I expect GT to end up at # 3 possibly # 2 on the Lifetime PS3 list. GT peaked about 10 years ago. Without significant bundling (which is possible) lifetime 6-7 WW tops. It would take years to pass MW2 and GTA IV on the PS3 all time list.