I think we can expect a reasonable 2-2.5 million out of Left4Dead 2, with the first dying off closer to 3 million.
Sequels to games released in the same generation often perform poorer than their predecessors in sales, and the fact that Left4Dead 2 is coming so soon after the first will only inhibit L4D2's sales even further. For an example of this, look at the Ratchet & Clank series on the ps2.
Only sequels that are able to differentiate themselves from their predecessors in some major way will manage to outdo their former in sales. An example of this is Halo 2, who's online component helped it to achieve amazing success.
Uncharted 2 will have such differentiating components, in the form of online competitive and cooperative multiplayer. However, I think Uncharted 2's success will fall more at the hands of marketing.
The first Uncharted was woefully undermarketed. Compare it to Assassin's Creed. Both are linear, story driven third-person action games, yet Assassin's Creed managed to outsell Uncharted by almost a million units on the ps3, despite the widely held belief that Uncharted is the better game.
Sony needs to start marketing their games like Microsoft. Microsoft doesn't just market their games as something good that people should buy. They market them as a good game that is also a BIG FUCKING DEALtm. They turn them into events that gamers may feel like they're missing out on if they don't buy the game. Gears of War's "Emergence Day" is a great example of this, and it was the incredible marketing campaign that propelled the game to an insane 36% attach rate over the '06 Holiday season.
Sony needs to start marketing their flagship titles in a similar manner. Instead of marketing them as simply good games ps3 owners should buy, they should be marketing them as games people should by a ps3 for. Uncharted should've been their own exclusive Assassin's Creed, but it wasn't.
Hopefully, Uncharted 2 will be given the marketing it deserves. Given Sony's past marketing efforts, however, I think they'll only bother breaking the bank on GT5, while shoving their other titles under the rug.
Depending on marketing, Uncharted 2 could sell anywhere from 2 million to 3.5 million. More then likely closer to the 2 million mark, sadly.
