I would probably approach it the same way I would approach a I, vi, ii, V progression. Regarding the iii chord, Em7 (E, G, B, D), I would either base a line around the Em7 arpeggio or the Cmaj7 arpeggio. I look at either as some form of the I chord (C maj9). If you are dealing with A7 instead of Am7 I would probably include a C# in the line, but I still think I would base the overall line on I, vi, ii, V in the key of C. And you could leave the C# out entirely to give that added tension with a line based in C Major as well.

[Edited by trebledamage on 12-15-2001 at 11:35 PM]