AutoWeek did an article a few weeks back on that subject.<ul><li><a href="http://www.autoweek.com/apps/pbcs.dll/article?AID=/20080124/FREE/42867745">Let it Snow...</a></li></ul>

I had 20" wheels with 610mm of summer rubber on the rear. White knuckles. Sweat and swearing. I would strongly suggest snow tires on 18" wheels if you are planning on driving in snow.

Boxsters, 911s (carrera, carrera 4, Turbos, all of them) are perfectly fine in the snow with snow tires, but are down right scary on summer tires in the snow.

I never bought snows, because of my delivery date, and nearly paid the price.

got nearly zero grip trying to get up an ice covered slight incline ..despite 4 powered wheels. On the visibly dry cement (but frickin cold since we got a lot of -25 to -30C) the car would be drift through all the corners in a seemingly slow motion. It was pretty wild yet fun in a devious way. My RWD BMW's (winter tires) didn't have issues with the incline or cornering.

Put winter tires on the R8 (or all seasons if you mild cold) and it'll be fine.

see the look on people's faces, when a sports car is flying past them, when their POS FWD or RWD car with all seasons are trying to get thru even an inch of snow