One could say that Bush wants Sadam out and put someone in his place that lets US economy benefit from the black gold in Iraq. There are various ways to do this, the only thing i can think of is that Bush needs someone in Sadam's chair that will bend over for the US.
On the other hand... the war is very expensive for the US (and i mean VERY EXPENSIVE).
But still, i think the main reason for this war is that Sadam is just a very dangerous man with alot of power ( for the next two to three days
).