Welcome! I have the same 2004 G 35 sedan, the original wheels are 7", I got a set o "take offs" from a 2007/08 sedan they are 7.5"and look great, then installed 235/50/17 Toyo proxes, you will not change ride comfort and your car will look great. The tire size calculator at:
http://www.1010tires.com/TireSizeCalculator.asphelped me a lot in size selection, go for it!
ocho