I agree with Coach C but would like to add one thing to it. If we didn't have a good RB I'd be all for drafting Bush, but we have a good RB in Davis (if we fix up our OLine like my suggestion in that thread, then he can be a top 10 RB in terms of rushing yards and a top 5-8 RB in terms of total yards). If we didn't have a decent QB then I'd be more open to looking at Vince Young or Matt Leinart, but neither of them are clear-cut stars for the NFL. Both have pretty weak arms, Young isn't especially accurate beyond 15-20 yards down field, and Leinart has mobility problems. David Carr is much more mobile than Leinart (he is slower straight-line speed than VY, but just as quick at dodging people), he has a much stronger arm than both VY and Leinart, and he is a more accurate thrower than VY. Drafting either Young or Leinart would not be much of an upgrade over David Carr (I don't think it would be any upgrade at all and more likely to be a downgrade, but that is debatable). I believe with competent coaching and playcalling and bringing in an OLine that will actually give him professional-quality blocking, David Carr can be at least a top 5-8 QB in this league, he is just like a more mobile, young Brett Favre, but to this point he hasn't been given the tutelage and appropriate personnel around him to improve like Brett did. Sure we have other picks, but using our top pick for a very slight at best upgrade at one position and relying on later picks to fill our problems is not a wise option, especially when other teams that actually need them will be drooling over these guys and we can make a trade to pick up extra draft picks this year, next year, and/or some veteran players and bring in 3-5 starting-quality players in exchange for that one pick, and in addition to the other picks we already have.
If for some reason we can't get a decent trade, then take Reggie Bush at #1 as he would be a more worthwhile use of the pick than Leinart or Young, but I think we'll get several good trade offers, especially if Young comes out too and adds that third star player. We need defensive help, especially if we switch to a 4-3 we definitely need to look for another DE in addition to CB/S help. If Mario Williams goes to the combine and runs his 40 in the 4.5-4.62 range and performs well, I wouldn't have a problem taking him at #1 overall if he looks like the next Julius Peppers, otherwise he'd be a good option after trading down to the #4-10 range. Offensive line is a huge problem for us and we need to look at getting a top OT, which are usually taken in the top 20 picks historically, so if we don't go for a defensive stud with our 1st rounder we need to look at the best OL available.
Bottom line, we have several gaping holes on this team, and rather than using later picks (maybe fill 2-3 of these holes with quality players with 2nd and 3rd round picks) we can trade the #1 and acquire additional high picks for this year and next year and/or get a coulpe veterans, so our two options are:
1) get 5-7 quality players in one offseason to plug into immediate starting positions (3-4 quality players from trading this one pick, plus the same 2-3 we would have gotten with our original picks anyways).
2) get 2-3 quality players to immediately plug into starting positions and one hopeful star that is a slight upgrade over our current players.
My personal preference is option #1.