I believe GTA: SA will be popular, but not enough to pre-order it. Plus, I'm sure that the local EBGames and GameStop will get more than enough copies, and let's not forget about BlockBuster, or your local game renting center. Block Buster must have fifteen extra Halo 2 copies just sitting there, and that's most likely what will happen to GTA. But, If you are a hardcore GTA guy, than you'll probably want to reserve it, just to make sure you wont be dissappointed.
-nF