Give it a bit longer to bump... it's only been 15 minutes.
I have GTA2 (which is topdown) for PC, and GTA3 for PC too.... but I have played GTA:VC and GTA:SA
I think 3 and VC are both really good games, 3 would probably be the best IMO, though VC is worth getting just for the bikes and helicopters.
GTA:SA was a bit dull I thought, and the "stories" ones are supposely more so.
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I think GTAIV will no doubt be an excellent game (whether it will be better than 3 I don't know) and due to the simultaneous release on two consoles it might sell close to what VC or SA managed when combining.
(I'm gonna say roughly 7 million each console... though 360 will have the bigger launch week sales)