18 front +19 rear or 18 all around is the best street setup, imo. 20's would ruin the ride/handling balance....cars don't handle well when their damping isn't specifically designed to work with very large wheels

 

A good set of WORK VS XX in burning black with a flush offset in 20" would look great.
A crappy set of chrome snow flake specials from Canadian tire in 20" will look like poo.

 

LOL @ "teen-age size rims"

 

20 will definitely work, but the look of 19 should be enough for the Z imo.

 

probably needs more than just rims and drop to work though...i have a tough time seeing a stock body Z running 20s...sample pics of HOT 20s

so yea 20s, IF DONE RIGHT, can look incredibly hot.