An easy way to tell a 325 from a 330 is that the 330 has chrome window trim, and the 325 has black window trim, unless it is a 330 with the performance pacakage.
The chassis and frame are the same between their respective convertible, coupe, and sedan styles, as far as I know. The only thing I think it different is the wheel base length on the M3.
Suspension on the 330 is sport already, as far as I know. The 325 you must order sport suspension (and the wheels and aero kit that go along with it) seperately. The sport option on the 330 consists basically of larger wheels and cosmetic aero enhancements, unlike the 325 where the "sport" pacakage is the suspension (shocks/springs/anti-swaybars) as well.
Standard wheels on the 325 are 16" and standard wheels on the 330 are 17".
This is all to the best of my knowledge.