Just bought a new 07 Cobalt LS coupe 5spd
I like the car but hate the factory RIMS and wheel covers looking to buy some aftermarket rims as soon as possible
my car has 195/60R-15 tires
wanting to go up to 16 inch rims with 205/55 R tires
wondering if this would affect my warranty

From what I've seen, as long as the outside tire diameter is the stock size so the mileage reading is accurate there shouldn't be any effect on the warranty.

However, I would ask my dealer for clarrification.

Yep. Changes in offset and/or overall diameter would most likely effect warranty coverage on anything that could be associated with changing from stock geometry. Check this page for sizing: http://www.miata.net/garage/tirecalc.html

Joel