I'm not sure if you understand tire sizing.

The first number (say the "245" in 245/45/1

is the width of the tire in millimeters. The second number (the "45") is the aspect ratio, or sidewall height. It is expressed as a percentage. For this tire, the sidewall height is 45% of 245mm, or 110.25mm. The third number, obviously, is the rim size in inches.

When changing sizes, you need to keep the overall rolling diameter the same. So if you've got a 235/50/18 now, you've got a rolling diameter of 27.25":

(235 x 50% = 117.5mm) x 2 sidewalls = 235mm + (18" x 25.4 mm/inch = 457.2mm) = 692.2mm or 69.22cm or 27.25"

You want your original new tire diameter to be within one cm of this to ensure your speedometer is accurate (unless you reprogram) and your front + rear diameters to be within 1 cm of eachother so you don't throw an ABS/low tire light. Your car likely doesn't have pressure sensor valves, it just works off differences in wheel rolling speed and the assumption that a low tire has a smaller rolling diameter and therefore rolls faster.

The 245/45/18 tires have a rolling diameter of 26.68":

(245 x 45% = 110.25mm) x 2 = 220.50 mm + 457.2mm = 677.7mm or 67.7mm or 26.68"

As you can see, it'* almost an inch smaller in diameter. Your speedo will be out slightly. A 255/45/18 would be a better choice at 686.7mm or 68.7cm if you're set on wider tires. They are almost exactly the same in diameter to your stockers and should be cheaper too buy and easier to find, too, as they are the stock front size on the Grand Prix GXP. The Potenza RE050As that come on that car (buy the GM spec tire, not the generic RE050A, they are specially wrapped to reduce torque steer and handle better) are amazing in my experience.