With the 17" GS400/430 rims, is it possible to put 235/50/R17. Even though the stock tire comes with 45 sidewall, i was thinking if i can put 50 sidewall for a smoother ride... The 16" rims (P215/60/R16) that came on my car are very smooth and quiet on the highway, so i wondering how i can maintain that ride quality with 17" rims...

 

Yes, you can. The difference is not much. I think is half inch taller. I'm sure because I went to PepBoys and aligned the 235/45/17 (what I have now) together with 245/50/17.
just a bit taller. Still I have one more taller. Can't find other sizes to compare.
Check here, its says all dimessions. Changing tire size affect the speedmeter accurace....

http://www.1010tires.com/TireSizeCalculator.asp

 

The diameter of the 50s will be 1 inch larger than stock. The speed reading will be 3.7% lower.

I would suggest putting more weight in choosing a tire known for good ride quality and comfort rather than simply bumping the sidewall size up a notch.

 

Stock tires:
235/45/17: Sidewall Height: 4.16 in 105.66 mm

New size:
235/50/17: Sidewall Height: 4.62 in 117.34 mm

Actual Speed: 60 mph 100 km/h : Speedometer1: 57.8 mph 96.4 km/h

Speedometer Difference: - Speedometer Difference: 3.672% too slow

(Just half inch diffe.) Compare the speedo variation

 

I am running 235/50/17 now and I am happy with it. Taller sidewall gives an extra cushion and no rubbing at all. I compare this size with 22/55/16 (stock) and the diff. is less then 2%.
I never regarded 235/45/17 as a stock size because 245/45/17 is a lot closer to the manuf. spec than 234/45/17