IF Hillary runs, she'll scare the shit out of most potential candidates except for Feingold with the amazing amount of cash she'll raise. The two will beat the living shit out of each other over Iraq, and a third dark horse candidate will emerge as the frontrunner in the 11th hour and take the nomination. Whoever this is will try to appease the Hillary-ites by appointing a woman as V.P. (i.e. Lincoln or Landreau), and try to appease the Feingold supporters by taking a firm anti-war stand.
McCain will also scare off most of his potential challengers with his name-recognition and war chest, but ultimately loose to someone more to the liking of the conservatives, and --if they're smart about it--with little ties to the Bush Administration.