Goodness...that would definitely fill the "lefty power bat who plays 1st base" need. That's a very tough call. I think that rotation circa 2007-08 needs Cain at the top:

Matt Cain
Noah Lowry
Kevin Correia
Brad Hennessey
(insert name of human being here)

But having Howard would solidify that cleanup spot for plenty of years. Wow. Insanely tough call.