I think you will see a slight spike, maybe even enough to beat the PS3 in the US for a few weeks, but not by much.
Everyone who would want ME2 already has an Xbox 360 so they could play Mass Effect 1.
I only think there will be a slight spike for 2 reasons.
1: The new 2 minutes commercial aired during football playoffs.
2: Xbox360 owners buying a new Xbox360 to replace their RROD Xbox360to play ME2.