The GT500 wheels are 18 x 9.5 and fit all the way around. As long as handling has a priority, you're better off with the same size wheels and tires on all four corners unless (until?) V8 forced induction enters the picture. You can get some pretty good tires in 255/45-18, 275/40-18, or even 265/40-18 if you don't mind a little more tire to fender gap. With a 4.0, you don't need more tire for most any purpose I can think of short of ESP-class autocross or serious road course open-tracking.
The very first thing - even ahead of wheels and tires - should be shocks and struts (I'm assuming that yours are probably 7 years old at this point). That alone will make the car's behavior a lot more composed. My own preference here would be for Koni Sports (aka "yellows").
What you'll want in stabilizer bars is for both to be adjustable. You'll have a better ability to dial the handling in to suit you and any future powertrain changes (the extra weight lives mostly at one end of the car).
Stick to moderate lowering only. Much past an inch, maybe an inch and a quarter starts becoming an "appearance first" choice that can be costing you some of what goes into "good handling".
There is a lot to be gained by opting for better brake pads, and up to the point where you're running hard on a road course GT-size brakes should be sufficient. Not to discourage you from the GT500's 14"-ers, though.
Performancewise, you'll be better off with a performance street pad that does not contain the word "ceramic" anywhere in its name. Think Hawk HPS or equivalent. You'll need something better still if you start road course open-tracking, keeping in mind that track pads are generally poor choices for street duty (they're dusty, noisy, and eat rotors).
'08 GT/5MT (mine)
'10 Subaru Legacy 2.5GT/6MT (hers)
'95 626/V6/5MT (autoX, winter, spare)
'01 Maxima 20AE/5MT (other spare)
Last edited by Norm Peterson; 07-08-2013 at 06:31 AM.