16" wheels require 195/45 R16 tyres.
If you want to go to higher profile tyres, then you'll have to change the wheels as well; IIRC on a 1.4, 15" is the lowest you can go due to clearance issues.
The ride will definitely be better on 15" wheels, but will still be limited by the inferior rear beam setup used on the pre-2010 model year cars. This is an almost exact copy of the Mk3 Panda beam and whilst it gives an adequate ride when used with 13" wheels & 155/80 R13 tyres, on 16" wheels and 45 profile tyres, it's going to be, well let's just say on the firm side.
The following extract is taken from the official Fiat accessories website.