Mario Williams will be gone by the 9th pick.
and i doubt Ngata will be available, Buffalo REALLY needs a DT, so Ngata isnt really an option. maybe he will be there if Buffalo fills in that need w/ a FA (Grady Jackson maybe?) and if that happens use him as trade bait. He is a top prospect, not a need for the lions, a need for other teams, trade down and get more picks.
Not only Buffalo, but Oakland and San Francisco need a big DT as well. Oakland has Sapp and Washington, and both are well past their primes. In San Fran, Anthony Adams isn't quite big enough to be a NT and Isaac Sopoaga doesn't have the athletic skill Ngata does.
If there aren't any trades in the top eight, then I expect Bush, Leinart, Young, Ferguson, Hawk, M. Williams will be gone with the first six. Either Oakland or San Fran (not yet determined) will have the seventh pick, and they may want a big DT, since there aren't many in this draft and none of them have Ngata's athletic ability. If Ngata is gone before Buffalo selects, I expect them to try hard to trade down. If they don't, they will likely choose Jimmy Williams or Michael Huff. If they trade down, they will do so with the idea of trying to still be in range to take Winston Justice or get either Jimmy or Michael somewhere later.