It took 360 eight weeks to get from 17 million to 18 million, so it'd take another sixteen at that pace till it hits 20 million but if you factor in GTA IV i think it'll hit 20 million in June.
08' Year End Predictions
Wii: 46 Million
Xbox 360: 26 Million
PS3: 22 Million
Sale Predictions:
Gears Of War 2 - 7 Million
Fable 2 - 3 Million
Left 4 Dead - 1.5 Million
Infinite Undiscovery - 700k
Tales Of Vesperia - 550k
The Last Remnant - 700k
Ninja Gaiden - 800k LTD [?] On its way.
Banjo-Kazooie N&B - 2 Million
Lost Odyssey - 500k LTD [X] surpassed
Too Human - 1.4 Million