There will no doubt be a big increase, but remember there is still Japan to drag it down. I say around 1 million first month. 800k second month, then it'll return more or less to normal #s of around 500k a month. This is worldwide of course.
It will not do as well as the PS3 slim because there is no pricecut, and 360 has been around a year longer.
Oh and by this I mean TOTAL 360 sales, new and old models.