I've been personally averaging 26_ mpg with half city and half highway. I think i could pull 30+mpg with a 325i
Don't forget to enjoy that sweet inline six though. Open her up whenever you feel like it, but improve your efficiency during your regular mundane drives. No need to redline and then slam on the brakes between red lights. You'll be surprised at how many people accelerate until the last second when approaching a red-light. It's ridiculous to see when you're maintaining an average speed through the streets.
I'll leave my house and meet up with other cars going the same direction, and i'll always meet them at a red light eventually. You don't really lose much time by driving slower.
I've personally not noticed a difference with the AC on or off. If anything the instant-mpg (which you should have on the display when driving "efficiently") goes from 45 down to 42 on a flat road with ac on/off.
If you're giving up air conditioning to save a dollar or two on a fill up, you should be looking at other ways of saving money, like selling your bmw.