CP3 has to be the goal until proven otherwise. Trying to strike gold in the draft or get a decent center are equally long shots. It's simple really, you go after the best possible player available that fills a need. CP3 fits that criteria and would give us the most complimentary 3 star lineup in the league. At that point you can go for role players, draft picks or whatever needs we must fill.
CP3 would make the position a lot better, but that position isn't a need for us (Billups).
If we're going for a superstar PG, Nash would make a lot more sense, because even if we TRADE for him his last year on his contract would be less. For let's say 1-3 years we would have a very fluent offense and giving us cap room for defensive players. Also, him and Dirk are the best shooters in the NBA. Having one of the best shooters, plus maybe the passer in the game doesn't hurt.
Heck, if we get Nash through free agency we could sign Billups as our 2-guard.
We shouldn't be looking to building a team to win NOW and several years in a row unless some amazing opportunity lands in our lap. With Nash/Billups or whatever, you can win NOW, and if Nash can't play that much anymore or whatever, guess what? Melo is only 27 and hasn't been injured most of his career. He's such a valuable superstar to have. It's not like if Nash can't play much after a year or two our team will be bad. In fact, our team could extend Nash's career.
This is more than Nash, hopefully you get my point. A max contract PG would be a desperate move to build for a team winning multiple championships in the next few years. Yes, you build for at least a few years, but you have to make sure it's not just Finals appearances and HOPE whoever is our center is enough to win four games. Those four games won't be easy with three maxes or not.
Do you think Chris Paul will improve a center's defense? Okafor is a good defender, and, well, ok, they played against the Lakers and the calls against him made him get the saddest look possible, understandably, but yeah. We're gonna need more than three max players. And if we do, Howard better be the third one.
Edit: Also, if Nash leaves in a year or so, we'd have cap room still. We're in a good position.