I'm hoping that David can pass Costa this week, and somewhat consistent results until Roland Garros might snag him a top 8 seed. He really only defends points in Estoril. Even if he loses early there, he lost early in ALL of the clay masters series as well as Barcelona last year, because he was unseeded and had to play people like Haas, Safin, and Hewitt in the first rounds.
Well it's a big gap to make up - Ferrero is some way ahead of him on points, and after all he reached a Grand Slam final last year where Federer didn't. Of course, there's an even bigger gap between Ferrero and the top two, showing how much they have distanced themselves from the rest of the field recently.
Federer may make it to number 3, but it won't be for a while, and it will depend a lot on how Ferrero does, as much as it will on himself..
Federer stand a good chance of being passed by Moya at the next two masters series. Ferrero defends next to nothing in Indian Wells and Miamia, as does Moya. Federer defends a final, and a third round. So unless he manages to win one, and go really far in the other AND Ferrero loses early in both, he's won't pass him.
In fact, unless I'm not considering something here, Federer has minimal chances of passing Ferrero untill Monte Carlo, but keep in mind that Federer also won Hamburg, so those two pretty much cancel each other out. The real "contest" for number 3 likely won't come untill Roland Garros, where Ferrero could repeat his finals feat and still lose it..
Also, regarding the "race" for number one... Well, there really is no longer a race. Unless Andre manages to lose in the first round of his next two tournament, he's number one. Hewitt has nothing to gain from either, seeing as he won both tournaments last year.
Agassi stands a reasonable chance of being number one after Scotsdale, and will almost definately be number one after Indian Wells. All he has to do is defend his points at Scotsdale and win a couple matches at Indian Wells, and there is nothing Hewitt can do to stop him.