Negative camber can help handling, but too much can hurt. Camber is just one
adjustment you can make to a suspension system to improve handling. The best thing to do is start with a stock suspension aligned to stock specs, that way you have a baseline to judge against once you start adjusting. If your car already has some suspension mods, align the car as close to stock as possible.

Next, take the car to some autocrosses so you can get a feel for how it behaves. Front-drive cars have a tendency to understeer because the front wheels are doing the accelerating, steering and most of the braking.

There are different things you can do to the suspension to control understeer, sharpen turn-in and increase overall stability and responsiveness. Here's a brief list of common adjustments and what they do:

Increasing rear roll stiffness - helps reduce understeer in FF cars, can also provide "lift-throttle oversteer," or the tendency for the rear of the car to "step out" when the throttle is lifted during a turn.

Increasing negative camber - On the front wheels, this can sharpen turn-in and can also provide some resistance to understeer, though not much and too much negative camber in the front can make understeer worse by decreasing the size of the contact patch. On the rear wheels, negative camber can help keep the rears from sliding out, but again - too much will decrease traction. **Too much negative camber will cause the tire to wear unevenly--the inside will wear faster.

Increasing spring rates - Can help overall responsiveness and stability in turns. The front and rear can be stiffened independantly to increase/decrease understeer. The general rule for FF cars being stiffer in back=less understeer.

Increasing damping rates - The dampers control the springs and keep them from bouncing excessively. Stiffer damping can improve the stability of the car over irregular surfaces and can also be used to control understeer (stiffen the back for FF cars like yours).

Toe - Toe out in the front makes turn-in sharper, but can make the car feel "twitchy" in a straight line. Toe in on front wheels can dull oversensitive steering, or lessen "twitchiness." Toe in on rear wheels can increase overall stability and lessen "snap oversteer." Toe out in the rear makes the back want to come around. **Any amount of toe other than 0˚ will increase tire wear. The further from 0˚ you go, the more tire you're scrubbing.

As far as a rear bar for your car, I have no idea, though asking where to start looking in the autox forum would couldn't hurt.

This isn't a detailed suspension tuning guide or anything, and I'm not a suspension wizard, but these are the basics I've learned from my experience and from people with much more seat time and expertise than me.