Depends on the sort. A naively implemented mergesort needs a temporary array of the same size as the the array being sorted. But whilst that takes up a chunk of memory for a while, it shouldn't be the sort of thing that makes GC choke.

And anyway, not many people will be using a naively implemented mergesort in this situation!

Sure, as long as you're okay with mutable data structures and the headaches they cause with threading.