The thing about MJ1 is that, the one thing it for sure does well no matter what you think of the entire movie, is set up Part 2 perfectly. I can't see the people that have seen the first 3 skipping this one (especially after seeing the trailers it should have), and those who skipped MJ1 in theaters will probably end up seeing it at home before MJ2 is out.
It may not beat CF, but I think 400 is quite doable. DH2 increased almost $90mil from DH1. It's certainly not impossible. PLUS, if it kicks as much ass as it should (and they fix a certain gripe), the WOM should be wild.