Can one give a conference talk on a topic already published in a journal? Let's say I wrote a paper and got it accepted in a journal. Can I then give a talk on that paper at a conference? What are the relevant factors here?
This may depend on your field and its customs.  In mathematics, it's totally acceptable and happens constantly.  

However, if the conference publishes proceedings (only a few math conferences do), you should not publish the same paper in the proceedings.  Some people use the proceedings paper to write a "survey article" intended to give the reader a broad view of their work and other related developments in the field, but they would usually not consider this to be an original research paper.

In other fields, where a conference talk is considered to be like a publication it its own right, this might not be acceptable.