I can see it doing RE4 numbers if the reviews and word of mouth are there.
It sold 1.6m on the Game Cube, and both the 360 and PS3 already have numbers around the GCN's lifetime userbase.
RE4 sold over 6m total, though nearly 2m of that was on the Wii, and may reflect "repurchases" for Wii owners upgrading to the latest version.
So I would put the floor at 4m. Ceiling at 6m. I'll split the diff and say 5m.