I want to change things so that if I select some text and start typing, the typed text replaces the selected text, similar to Motif.
You want to use something called pending delete. Pending delete is what happens when you select a region (with the mouse or keyboard) and you press a key to replace the selected region by the key you typed. Usually backspace kills the selected region.
To get this behavior, ensure that you have the pc package installed, and add the following lines to your init.el/.emacs:
(cond ((fboundp 'turn-on-pending-delete) (turn-on-pending-delete)) ((fboundp 'pending-delete-on) (pending-delete-on t)))
Note that this will work with both Backspace and Delete. This code is a tad more complicated than it has to be for SXEmacs in order to make it more portable.