If by "Almost Everyone" you mean at most 1/3 (probably closer to 25%) of the XBox 360 or PS3 userbase you might be correct ... There are no games that have a truely universal appeal anymore, and few games interest more than 10% of gamers in general.

I could be wrong, but I suspect that some people have become overly optimistic about the sales of Grand Theft Auto because of how well it sold on day 1. One thing we have been seeing from a lot of big PS3 and XBox 360 games is heavily front loaded sales, Halo 3 (as an example) sold 50% of its total lifetime sales (to date) in its first week and nearly 70% of its sales (to date) in its first month; certainly, Halo 3 will continue to sell at a steady pace for the rest of the generation but it will be a very long time before the first month counts for less than 50% of its sales.

We won't know for a long time, but it is plausable that GTA4 have a similar level of front loaded sales as Halo 3; and it is possible that the front loading could be much worse. To demonstrate this, how many big GTA fans do you know who haven't bought a PS3 or XBox 360 yet? Personally, those I know who "Love" GTA bought a PS3 or XBox 360 months ago to play other games (Bioshock, Halo 3, Call of Duty 4) because they knew they would be buying one for GTA4 anyways.

Generally speaking, games that deliver incremental upgrades to past experiences don't attain the same sales levels as their predecessors. While GTA games may seem to defy this in that each installment has sold more than the previous, you have to consider as well that like most games, their lasting appeal is not that great. A week or two after a new GTA comes out, the past iterations drop off in sales dramatically as they become seen as inferior to the latest one by prospective buyers. And of course, thanks to gamer drift, some people end up dropping the series after a given iteration.

These two factors combined ensure that games like GTA don't actually sell much more than their predecessors even in a best-case scenario. In terms of the userbase at the game's launch versus continued sales after the newest iteration drops the game from its perch as "GTA of choice", each GTA has managed to sell to progressively smaller amounts of the installed userbase at the time of their GTA-series dominance.

 To illustrate:

GTA3 ~ 11.60m on PS2
Sales Dominance: 10/23/01 to 10/28/02
Userbase Peak: 33,882,237 (~34% of userbase)

GTA:VC ~ 14.20m on PS2
Sales Dominance: 10/28/02 to 10/26/04
Userbase Peak: 70,961,023 (~20% of userbase)

GTA:SA ~ 15.45m on PS2
Sales Dominance: 10/26/04 to 04/29/08
Userbase Peak: 117,345,554 (~13% of userbase)

 This is not 100% accurate, of course, but as games with front-loaded sales, it holds a certain degree of accuracy.
 

This is seriously entering the realm of speculation, but GTA4 sales can be projected using the above examples (cementing this as a less-than-reliable technique, I might add), by assuming that the next GTA will arrive within 2 years, and that the userbases of the 360 and PS3 will grow according to the general sales curve they're following.

Scenario 1: GTA3 sales levels

GTA4 ~ 11.90m on PS3
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 35,000,000 (~34% of userbase)

GTA4 ~ 10.88m on 360
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 32,000,000 (~34% of userbase)

Cross-Platform Sales ~ 22.78m


Scenario 2: GTA:VC sales levels

GTA4 ~ 7.00m on PS3
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 35,000,000 (~20% of userbase)

GTA4 ~ 6.40m on 360
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 32,000,000 (~20% of userbase)

Cross-Platform Sales ~ 13.40m


Scenario 3: GTA:SA sales levels

GTA4 ~ 4.55m on PS3
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 35,000,000 (~13% of userbase)

GTA4 ~ 4.16m on 360
Sales Dominance: 4/29/08 to 4/26/2010
Userbase Peak: 32,000,000 (~13% of userbase)

Cross-Platform Sales ~ 8.71m

As a projection, this is largely like shooting in the dark in the general direction you think your target might be in. So please, whatever you do, do NOT take this as an authentic prediction or even remotely reliable estimate.

The issue is that I think certain people forget that San Andreas is still charting on the VGC Top 200.....

Fact is, the 360/PS3 userbase is only going to grow, and buy GTAIV as a "staple game" in the same way that users are continuing to buy specific PS3 games and X360 games (like COD4 and H3).

Mark my words: GTAIV is going to sell well over 13m this year alone, and do rather well next year.

GTA4 is only a "staple game" as long as it has no sequel. The moment GTA5 (or whatever they decide to release between GTA4 and GTA5) comes out, GTA4's sales will fade the way GTA3's did after GTA:VC came out (and GTA:VC's did after GTA:SA came out). This is the key weakness of sequels: Succession Syndrome.

According to Take2 figures ( sales to retailers ):
GTA III : 14.5 M
GTA VC : 17.5 M
GTA SA : 21.5 M
GTA LCs : 8.5 M
GTA VCS : 4 M

http://taketwovalue.com/documents/TTWO_Value.pdf#page=12

I think GTA IV LTD will NOT outsell GTA SA and I doubt even that it will outsell GTA VC.

Back from the grave

GTA IV has shipped 13 million as of now.
http://news.vgchartz.com/news.php?id=3119

Looks like it should outsell GTA3, but I am doubtful it will reach Vice City.

I presume all of the shipment figures in Celines 2 posts include the PC version.... so they are on pretty equal terms.

The last official numbers from R* (this is about a year ago now though) regarding previous GTA's were:

GTA III - 13.5 million
Vice City - 14.5 million
Liberty City Stories - 8 million
Vice City Stories - 4.5 million
San Andreas - 21.5 million

(these were for all versions/platforms combined)

So it will outsell GTA III, possibly Vice City. I highly doubt it will outdo San Andreas.

Source: The numbers were released as part of a briefing to shareholders to get them to reject EA's offer. You could download the pdf previously but the link on wikipedia is now broken, you may be able to find them somewhere else. The number for San Andreas is in Guinness World Record Book as well.

Celine posted higher figures for III and VC above.

Perhaps the ones you posted don't include PC and/or xbox versions.