i have another car, that was running on 17s as standard, the ride was firm in the first place as the car is a sporty model and had quite tough suspension.
i changed to 18s a couple of years ago and the ride got a bit harder, the tyres are 235/40/18s
i had a set of 19s fitted for a while, which were running 235/35/19s and the ride was seriously hard to the point where the cd would jump on a bad road. but it went around corners like a train!!! (it didnt bother me so much but the wife hated it!!)
i changed back to the 18s and sold the 19s to a m8.
ive found its a trade off really, the thinner the tyre the harder the ride but the better the car handles/looks.
put it this way, if when i fit my 18s to the A4 its tollerable im going straight to 19s!!!!!
my A4 is at the mo running on 16s, i will be selling my other car shortly to get an A6 2.5tdi, i will be fitting the 18s to the A4 so i'll let you know the difference then.