I actually decided to cheat a bit by just borrowing what I needed from the library and not returning them until the end of the semester. Late fees were ten times cheaper than buying the books, even if I could manage to sell them again after I was done with them.
Anyways, Anski's spot on. Depends on your class, professor, what book it is, etc. In general though, it's probably best to take your time to find the exact edition that's required, since you never know what might be different from one edition to the other. Some "editions" are actually entirely new volumes.