from what i hear, auto setting does a poor job at automatically adjusting the fan speed when the card gets very hot. I usually leave my fan on 50% on my nvidia card using evga precision. If you can stand the noise, then leave it on manual and adjust it to what you want. I am sure there are programs that can do a better job at adjusting the fan speed for you, I do not know of any ati ones atm.

TLDR: leave it manual and adjust the fan higher when you game and lower when not gaming