If Omega is only able to predict your answer with 75% accuracy, then the expected payoff for two-boxing is:
.25 * 1001000 + .75 * 1000 = 251000
and the expected payoff for one-boxing is:
.25 * 0 + .75 * 1000000 = 750000.
So even if Omega is just a pretty good predictor, one-boxing is the way to go. (unless you really need a thousand dollars or usual concerns about money vs utility)
This seems an overly simplistic view. You need to specify your source of knowledge about correlation of quality of predictions and decision theory prediction target uses.
And even then, you need to be sure that your using an exotic DT will not throw Omega too much off the trail (note that erring in your case will not ruin the nice track record).
I don't say it is impossible to specify, just that your description could be improved.
EDIT: I see by the karma bombing we can't even ask. Why even call this part of the site "discussion?"
Some of the classic questions about an omnipotent god include