I don't think there's anything magical about being in the top 20 IM programs in the country. Things that are important include being academically inclined and research oriented. Whether or not you intend to stay in academics or do a lot of research after you enter hem/onc, it always helps to show interest by doing these while you're in med school/residency. It certainly helps if your research mentor is a well known hematologist or oncologist, because as with any job interview, who your references are make a big difference.