Ferrari will dominate and Rubens will improve to finish 2nd alot.
Biggest improvement in my view will be McLaren/Toyota
I think BAR-Honda will not be as competitve as last year and that Williams will improve and challenge McLaren.

So in my view the constructers in a guess would like this:
1/ Ferrari
2/ Renault
3/ Williams
4/ McLaren
5/ BAR - Honda
6/ Toyota

Remembering that Renault has alot of pressure on them, but i could see McLaren finoishing second with two of the best drivers and most different drivers.

Hard thing to say!