Hi Maverick!
I think the correct option is 1) he has sold till now over 2 million CD's. I'm not sure whether we can say the other two in these cases. I'd use the plural form (millions) in a context such as this: He got (eight) millions for that movie, where millions is nominalized and stands as well for >"dollars, pounds, etc." Another example of this one: Millions were killed in the battle > Again, millions is nominalized and any of the words "people, soldiers, men", ellided. Then, the last option, I'd use it in this kind of context: Millions of people were at the party (rather exaggerated, uh?). So, in this case, it is the subject of the sentence and not premodified by any elements.
Regards,
Mara.