All pitch shifters are not created equal. What you want is a type called a formant corrected pitch shifter. The cheap "voice changer" programs you find here and there are not formant corrected.

Waves UltraPitch:
http://www.waves.com/content.aspx?id=288

This one is for Mac:
http://www.soundtoys.com/products/PurePitch/

Here's one as a hardware device that's supposedly pretty good:
http://www.bossus.com/index.asp?pg=1&tmp=59