I went the same route as SMARTIEPANTS with the 6 inch RIALS front and rear and used the stock rear tires on the front and the 195 series on the rear. I also bought a set of 5 inch wheels from Tirerack with the hopes of mounting my stock fronts for use as spares.
I drove them around for a couple days in town and thought the (4) 6 inch wheels were fine. Then I got on the freeway and as soon as I got to 75 MPH the car felt VERY UNSTABLE. Each time I corrected for lane irregularities the car felt like it wanted to continue turning. 70 mph was now the new top speed. I returned home and put the 5 inch (spares on) and stability returned to 90 MPH.
I then went back to DISCOUNT tire and had them change the front 175's from the 6 inch RIALS to the 5 inch Rials and the car handles now as well as it did stock.
DO NOT PUT 4 6 inch RIALS on a smart !!!!!! IMHO
Thanks,,,Alan Travis