I recommend you buy a used 350z suspension for 150-200 bux from the guys at
http://www.my350z.com. this will give you a fairly dramatic change. from that minimal investment or from Q45 tech's advice. I like his advice in this case, IF you don't want the lowered look. A lot of G owner's will not want to lower their car.
I have the Z suspension on my car and have had it for about 15,000 miles now.
either one of these presents minimal investment and allows you to decide afterwards if you want to go more or less.
benefits of the adjustable sway bar: it is adjustable, and does not lower your car
benefits of the Z suspension: shock valving is more aggressive, lowers car, cheaper.
the adjustable sway bar has no drawbacks
the z suspension may be too firm and promotes mild porpoising on rough roads. but you can take it off and go back to stock in about 3-4 hours.