C-h n will give you the NEWS file, explaining all the changes in the latest version of Emacs. In there you will find the answer you are looking for (how to get the old behaviour back):
** Changes affecting display-buffer
*** display-buffer tries to be smarter when splitting windows.
The new option split-window-preferred-function lets you specify your own
function to pop up new windows. Its default value split-window-sensibly
can split a window either vertically or horizontally, whichever seems
more suitable in the current configuration. You can tune the behavior
of split-window-sensibly by customizing split-height-threshold and the
new option split-width-threshold. Both options now take the value nil
to inhibit splitting in one direction. Setting split-width-threshold to
nil inhibits horizontal splitting and gets you the behavior of Emacs 22
in this respect. In any case, display-buffer may now split the largest
window vertically even when it is not as wide as the containing frame.
This is for 23.3, for 23.2, you may not have the option of setting it to nil (judging by the wording above, this was added at a later date) but setting it to a very large value (more columns than will fit on your display) should also work.