I am going with 19" wheels because I want looks and performance. I was also thinking about either 275 or 265 for the rear tires and 245 for the fronts.
My question is, I dont know about all the offsets and things like that, and I was wondering if someone can look at my setup and help me with what I want to do. Like how do I know if some rims will fit the tires I am getting.

 

To keep your tire setup as close to staock as possible in 19's you should run 245/35 and 275/35 front and rear respectively.

A 245/35 can be put on a rim anywhere from 8-9.5 inches in width.

A 275/35 can be put on a rim anywhere from 9-11 inches in width.

The stock offset for the Z is as follows

17's 30mm front and rear
18's (non Track) 30mm front and rear
18's (Track) 30mm front & 33mm rear

Most people think the stock wheels sit too far in the fender well and want their aftermarket wheels pushed out a bit, so they go with lower offsets. Anything from 20-30mm should be good. Just remember. The wider the wheel gets the higher your offset can be.

Here's a good tool to use in order to determine the correct offset.

http://toy4two.home.mindspring.com/offset.html