Just to put things into perspective:
The Wii needs to sell 4.42 million before the X360 sells 2.72 million. That's the same as selling 62.5% more.
For 2009 as a whole, the Wii sold 109% more, and in the holiday quarter that was 130% more.
There's very little doubt that the Wii willl make 70 million before the X360 makes 40 million. In fact, it's so obvious I would call it a somewhat poor prediction.
Your "Wii will hit 70 million by March" is rather good, though. If it sold as well as it did last year, the Wii would make it by 50K (excluding future adjustments of previous data). Essentially, your prediction would be that Q1 2010 would > Q1 2009, which is rather interesting.