I'd caution you not to. If you are sure that you want to start the Sanchise against Buffalo, I'd look elsewhere on your team for dead weight worth dropping for the Jet pigskin thrower. Carrying three QBs can be scary, but you could easily drop the Sanchise after this week (or McNabb if he gets injuried, which is a likelyhood at any time behind his often porous O-Line.
I'd think that McNabb would get snatched up really quick off waivers, depending on your league size, and you wouldn't like to lose to him later in the year on another team.
Speaking of dropping a 'Skin, check out my idea on dropping Portis: viewtopic.php?t=477570