A few more variables will affect sales besides the quality of the title (which given Sega's recent history is very much in question).
First and foremost is when this game comes out. The more consoles that are out there at the time, the more potential sales. Then again, if this game comes out near Super Mario Galaxy it's going to have a tough time competing with what is shaping up to be a Nintendo AAA title.
Second will be marketing. I believe Nights has a huge video game enthusiast following but isn't very well known to the general public. Since the Wii caters more to the casual game player, Sega will need to make sure to get placement in front of those people. Hopefully Nintendo will have a 'Demo' channel in time for this game. If Nights 2 is good and people can get a taste of it for free (a la Christmas Nights), the sales could easily blow away any of our estimates.
In the end though, I expect Nights 2 will be somewhat better than Sonic and have sales to match.
(As a side note to foolflexible, Sega was a threat to EA's bread and butter -- their sports games. By locking up many of the professional licenses, EA basically forced Sega to stop making those games.)
Numbers are like people. Torture them enough and you can get them to say anything you want.
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