Can 20' inch rims fit on a 2005 infiniti g35x sedan and have a smooth ride

No.

It will be so hard that your fillings will fall out.

You can fit it on there, but it will not be a smooth ride at all. It is gonna be a lot worse than your factory suspension feels, as in your gonna feel every single bump in the road and be bouncing up and down a lot most likely. 20 is are too big for a G35 sedan if you ask me, they do look good, but l would not trade the look for a smooth ride.

20'' rims look great on ur car

http://cardomain.com/member_pages/show_image.pl?image=http://memimage.cardomain.com/ride_images/3/2788/461/31967730012_large.jpg?064209-330
http://cardomain.com/ride/3028265

there r lots of wheel choices

http://tirerack.com/wheels/results.jsp?filterSize=20&filterFinish=All&filterBrand=All&filterSpecial=false&filterNew=All&autoMake=Infiniti&autoModel=G35+Sedan&autoYear=2005&autoModClar=Automatic+Transmission&sort=Brand

you will have to mount them on low profile performance rated (W or preferably ZR) tires.

the ride will be firm as a result of the low profile tires (nil to do with the wheels) but still smooth & likely more in keeping with a sports sedan.

..why not just take a ride with someone who is riding on very low profile tires before deciding on this upgrade or get on an infiniti owners forum (lots of infiniti owners have rescaled their wheel sizes) & post the question there?

http://infinitiforum.net/

cheers (19'' foose rims & michelin 295/30 ZR 19's)