San Andreas sold 8.5 million copies to date. I don't see Halo 3 passing it.
Probably not, though GTA SA is not a good measuring stick, since it was sold for the PS2 as well as the original XBox, which gives reaches 20x the base that Halo 3 is being sold to.
For the other poster who asked, Halo's campaign has always been decent, but the true strength of the games has always been the multiplayer aspects of it. That's what seperates it from typical FPS. BioShock is definitely the way to go for solo gaming, if that's what you're looking for.
H3 will have all its bugs ironed out through updates and patches, and will eventually become the best online FPS ever. Bungie projects this to have a 3 year life, so there will be constant updates and new content over that time period.