The Korean Hangul IME which was released with Windows 2000(?) uses the right ALT key to switch between hanja/ascii input. The switch key is since then obsolete. Someone please correct me if that is wrong, my Korean knowledge is minimal.
There are three(? honestly I am not sure, because I didn't use Windog for everyday) kinds of methods. What you said is first one, we call it type 1. (Actually it serves to input korean language using non-korean keyboard.) In type 1, right ALT represent the switch between hangul/english, while right CTRL for hanja/hangul.
IMHO, type 3 is slightly better beacause I can use right ALT, and CTRL. In type 3, switches between hangul/english and between hanja/hangul were shift-space and control-space respectively.