It should be able to break 5 mill if Bethesda put enough advertising behind it, the developer pedigree is there and the game looks more than solid. It should as you say review well...
On the other hand this year is very crowded with AAA titles especially in the FPS genre and it releases between Resistance 3 and Gears of war 3 and both Dues Ex: HR and Batman: AC launch within a month ether side of the game this could hurt the initial launch of the game so it will probably be relying on good legs and strong PC sales to get it past 5 mill.
I mean Doom 3 between PC and Xbox apparently sold 3.5 million http://www.eurogamer.net/articles/sales-show-doom-iii-was-not-bad-willits so 5 mill over 3 platforms with 2 console platforms that are both much healthier than the Xbox and stronger DD and MAC/Linux gaming growth on the PC side, should be possible even if it doesn't carry the Doom brand, I am sure it will have from the makers of Doom and Quake prodly displayed on the box anyway...
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