im trying to decide to get either an 850csi or an 840csi the only reason i would get an 840csi is because of the gas milage. but wht does everyone else think? there is a huge difference between these two cars.

I would definitely go for 850. The consumption difference is not that much, and from 850 you get more refined, more sporty, more comfortable car.
P.S. go manual if you have option to do so.

There is no such thing as an 840CSi.

Are you referring to a "regular" 850i or 850ci or are you actually looking for a rare 850CSi?