Offhand I would say average or better would give you a pretty good shot, as long as the rest of your numbers are in line. I think this goes for pretty much any mid-tier academic IM residency. Higher is always better if you haven't taken the test yet.
I interviewed at UC, and turned down an interview at IU with an average step I score. I feel confident that I could have matched at either place.
If you want to stay in that area, you might look at OSU as well -- I liked it better than UC.