On street tires on my GSR I run 38 in front an 35 in back when hammering on it. Get all the crap out of your car and run with less gas in the tank. Wait on the suspension mods, read up on what you can do before you get yourself into a tougher class. Consider aftermarket brake pads too, you will be stunned at the difference that can make. Keep in mind that aggressive pads will eat up your rotors a bit quicker.

If you get one of those laser thermometer things, you can read the temp of the tire, which would help you on the tire pressure, you want even temps across the tire. Hotter in the center, too much air, hotter on the edges not enough.

No offense intended, but the best money you could spend would be to do a good driving school.