I'd suggest allowing multiple entries per artist. There's no real rationale not to, from what I can see. If we even manage to get this list to 200 (kind of slow going right now), we can always debate the merits of cutting someone's third or fourth entries off it. I'm not about to nominate 10 songs by a single artist but it'd be nice to include, you know, three Beatles tunes or a handful of James Brown songs or Coltrane tracks or...you get the pic.
A few off the top:
Association: Never My Love
Brenton Wood: Gimme a Little Sign
Al Green: Back Up Train
John Coltrane: Alabama
Cream: Sunshine of Your Love