I would like to lower my G35 and put some 20s on it. How much can I lower the car with out problems?

I will get some staggered wheels 20x8's in the front and 20x10s in the rear.

 

With some tein s.techs you will be perfect.
I have some for sale $165, if you are a sedan you might need to roll your fenders.

 

I have a coupe, how low am i going with those? 1 inch, 1.25 inch ... ?

 

about 1.5 front 1.3 in rear for your coupe.

For my sedan they were 1.9 front and 1.7 back.
Pm me if your interested =)

 

do you also have wheels on your sedan? if so, what size are they?

 

Doing a lot of research and searching on this site is your best bet but...
A lot of coupe owners end up getting the Tein Htechs for 350Zs.
The spring rate matches most closely with the OEM sport suspension.
And the 350Z h techs usually tend to lower the car less agressively and that means less premature tire wear due to neg camber.
Usually don't have to worry about camber kits with the Htechs either.
But that doesn't mean you need a camber kit with the Stechs, it's just advised due to tire wear.
It's basically the least expensive route to go.

 

I was running 20's with no rubbing problems. Before i took them off i had 18's on and they still looked great. I have pictures here...