If you widen the front track of the car via spacers, you will REDUCE understeer.
If you widen the rear track of the car via spacers, you will INCREASE understeer.
Do BOTH at the same time the car would be able to pull more lateral G's, but the oversteer/understeer relative to eachother would negate eachother. So the car would grip more, but you would have about the same understeer/oversteer characteristics.
About how it looks it completely up to you. I personally think that a flush wheel/tire looks better than a "sunk in" wheel.
You'll need the spacers, and a quality set of wheel studs or longer wheel bolts. It's a relatively inexpensive mod.
More info here:
http://www.1addicts.com/forums/showthread.php?t=12647