I think this one is on the fence, one thing not taken into consideration is the average of gamers.

5 years ago gaming was still primarily teenagers, so people that depended on parents to purchase the game.

Those ppl have grown up and you still have the huge teenage base. More people can just go outright and purchase the game.

Considering GTA IV seems to have a lot bigger opening it is far from certain it will have similar legs.

 Why? There are stock issues in many places, and with rapidly growing install bases, there is going to be a much larger PS360 total install base by the end of the year, and most of those are going to want to play GTA IV, simply because it is the best game of all time (according to gamerankings and metacritic) so if anything, it will have more legs.

I see GTA IV landing somewhere in the 16-18 million range.

of course it will, there were tons of people waiting for this game. not to mention that it launched on two consoles, first week sales are going to be huge and GTa has legs.

Its hard to say ...

A large portion of people who owned previous GTA games (potentially the majority) do not own an XBox 360 or PS3, and may not ever own an XBox 360 or PS3; on top of this there are many people who have owned previous Grand Theft Auto games who will choose to rent (or simply not play) Grand Theft Auto 4 for a variety of reasons.

Now, beyond getting former GTA owners to buy an XBox 360 or PS3, this means that people who have never played a GTA game have to buy GTA4 which seems like a very large challenge; because a game that has sold 15 Million copies has hit a saturation point where most of the people who haven't bought the game have little or no interest in a game like it.

One thing that GTA4 has going for it is there was rampant piracy of Grand Theft Auto games on the PC, XBox and PS2 and (between Blu-Ray and XBox Live) piracy on the XBox 360 and PS3 is less desireable or much more difficult.

It's too early to tell; sure it had a great opening week, but the only thing I'm worried about is the legs it'll have. But I do see GTA IV outselling the previous ones.

The piracy issue is a legitimate point that should translate to higher overall sales, but will probably be more than offset by the overall current smaller user base of both the 360 and PS3 combined. I don't think the reduction in piracy will result in millions of extra copies being sold. Still, I expect to see higher sales over time as the 360/PS3 user base continues to expand with a high attach rate and protracted overall sales.

Although the smaller user base is not reason enough to believe GTAIV will sell less than GTA:SA, keeping in mind the attach rate of SA at the time. It was significantly lower than the current attach rate on either console today. Of those 100+ million consoles, how many belonged to consumers not in the GTA demographic? Quite a few.

I think GTAIV will have great sales but be limited by the HD consoles limited base. I think IV will outsell III but never reach SA sales.

If I can recall correctly San Andreas was also on XBox (and I'm sure I am correct as I owned it but never really played it as didn't like it so gave it away).  If you are going to combine sales of GTAIV on PS3 and 360 shouldn't you combine sales of San Andreas on PS2 and 360