Even if he defeats both Klitschkos, Haye will never go down as an all time legend. The best of his generation certainly, which is the utmost he can achieve, but its a generation that has been devoid of any real stars since Lewis, Holyfield, Rahman and Tyson all quit. The heavyweight division is full of alsorans at the moment.
If Haye loses to one Klitschko, his heayweight career is effectively over as well. He moved up from being the world's best light heavyweight, and the only place a boxer of that calibre belongs is at the top. After defeat, there's no point in continuing - look at Ricky Hatton.
So, ideally for him, he'll fight Wladimir and win 2 more belts in the autumn. He can then wrap up the heavyweight division in spring by beating Vitali. He'll then have a defence against the no 1 contender in autumn 2011, and if he wins that, its hard to see why he would need to continue boxing unless an amazing threat emerges in the next 2 years. After all, he would already have beaten everyone worth fighting in the division, and could retire knowing he's done all he can.
Alternatively, he could do what Calzaghe did and keep going for years, defeating everyone who thought they were good enough to have a go.