For somewhere like the Goucher program, I'd ballpark things as
3.8-4.0: Great
3.70-3.79: Good
3.50-3.69: Acceptable, but will probably limit some options
3.00-3.49: Not horrible if nothing below a B, or maybe one B-. Definitely more limited options, factors like undergrad GPA and story will need to be much more emphasized.
<3.00: probably SMP time. Given other factors (the kind that get you into a one-year postbac in the first place), DO schools might still be an option without an SMP.
The bottom line is that you don't have to be perfect, and even if you disappoint yourself, the postbac still considerably increases the odds. For instance, several linkage options are pretty lenient, and will offer significantly improved odds for those worried about their grades. MCAT prep definitely helps, since these schools usually still require at least a 30-32 MCAT to compensate for the grades.