In order to have WoW stop streching your screens, you need to go into your WoW folder/WTF there should be a config file in there. Open it with notepad and change the resolution to whatever you'd like.

I'm at work but i think the string in the config file is something like "SET gfxres 800x600"

I'm sure I'm not really close but that should give you something to work off of.