Paper accepted for poster presentation in IEEE, what are the chances of it being published?
It depends on the particular conference and the field.  In computer science (where the main publication venue is conferences), many conferences divide the published papers into "talks" and "posters" (eg NIPS, AISTATS, ...), and there's no difference in terms of publication.

In most other fields, however, acceptance at a conference means nothing, and you have to get published in a journal.

TL;DR: ask your advisor.