In my opinion Renault brought so many new elements during the second half of '08 that they will have to work hard during this winter (as McLaren and Ferrari...but they had a reason: fighting for the world championship).
I expect McLaren and BMW to challenge for the first races of 2009, and maybe Ferrari and Renault joining them after 5 or 6 races.
Even Vettel, if Red Bull gives him a decent car, can fight for podium positions...and I expect Honda and Force India to score some points more.
So maybe Renault will win 2 or 3 races with Fernando...but it would be hard to fight for the championship. (constructor's championship will be even more difficult, with Piquet, who is not able to bring so many points)