think of a sway bar as a spring that links your front left/right suspension and rear left/right together in pairs.

since you say your car is already a bit rotation happy, then i would advise against picking up a rear sway only, but then again, i've always leaned towards setting up suspension for higher speeds. It will only make your car rotate more, but apparently that's the fad these days because it handles pretty well at low speeds (i.e. daily driving and auto-x), but once your speeds get up there (i.e. road course racing), it's pretty damn hard to control.

So I guess my point is, set it up for whatever you're going to use it for, but be aware of the limitations. For road racing, you don't want too much oversteer, but for auto-x you want alot.