L4D2 will be seriously hurt by MW2, the fact that it's only been a year since L4D1, and, honestly, L4D2 doesn't look like it adds a ton to L4D1, so I'm not really sure if I, let alone more casual buyers, would feel compelled to buy it.

Assassin's Creed, I really don't know. I'm still amazed at the insane sales for a new IP. I can't remember if AC1 had a lot of hype, or was more of a sleeper hit that relied on word of mouth. I'm sure that will make a difference. AC1 also had the benefit of releasing when there wasn't as many good games for the HD systems, while there is now.

Assassin's Creed II is extremely strong.

12 million lifetime. (7 million on X360, 5 million PS3)

If anything, it's Assassin's Creed II who will hurt CoD 6 because ACII comes out first and people will be busy playing it when CoD 6 comes out.

^^ not to mention there hasn't been an AC game since 2007

It will only affect their chart performance.

i think L4D2 will be a little affected by it but not a whole lot

l4d2 will be.. or at least where i live
ac2 wont sell anywhere near aswell as the first because all the people i knew who bought ac sold it after they beat it and felt like it was a waste.

More  like 6 million lifetime 4 million 360

2 million ps3

LFD2 is a valve game. It'll be fine. And awesome.

AC2 may be hurt a bit though.

I'll take a bet it wouldn't even come close.

Neither. Maybe L4D a little bit, even though it's a very different kind of FPS, but Assassin's Creed 2 is in a completely different genre, so I don't see why Modern Warfare 2 would affect it at all.