I'm guessing Merom at least to start with for the imacs...they'd need a totally new board for Conroe.
For the Mac Pros, here's what I'm thinking:
Low end: Conroe XE 2.93ghz
Midrange: Woodcrest 3ghz
High end: Dual Woodcrest 3ghz for the Quad.
Then by this time next year, move it up. Low end either the 3ghz woodcrest or the conroe XE, midrange either dual woodcrests for the quad or single Tigerton quad-core, and for the high end Dual Tigerton quad-core chips for an OCTO!
Then in 2008 have Dunningtons...rumors say this may have between 4 and 32 cores per chip! I hope its 8....