Look at today's antique market for a guide. There are quite a few 330ci on the road, so it won't be rare like a Camaro RS, Mustang Shelby, Sting Ray, etc.
It seems that the car has to be popular, a bit unique, and at least 25 - 30 years old to start going back up in value. Of course, if it has LOW mileage and showroom condition, it make a positive $$ difference, but not with 150K miles.
Now, my personal opinion (which is probably full of crap!) is that the M cars will ultimately start going up, and MAYBE the 330ci CONVERTIBLE, as they are much less common.