Sabik - your car uses the 70 amp alternator. Later years used the the 80 amp unit (I believe >1990). The wiring harness is different for the two alternators, so they aren't readily interchangeable. As with anything, you could make the 80 amp alternator work, but just keep in mind that you even though you get more maximum current out of the 80 amp unit, your current-at-idle will suffer somewhat (could be a problem idling with stereo, lights and ac on). So you will be getting more power, but it will be at a higher engine-speed.

The other important consideration is what flyonwall9 brought up -- Are you sure your unit needs to be replaced? My alternator wasn't charging, so I brought my car to a shop and they quoted my $150 for a rebuild. I decided I would try my luck with a set of new brushes even though mine didn't look old, and three years later it's still working.
Best $12 I ever spent on my car.

Peter
1988 420sel
1988 300e
2001 X5
1999 S70 AWD
1997 960 Wagon
1996 850
Arriving soon: 2002 530i