Found at http://home.earthlink.net/~engarcon/e21digest.htm

The stock E21 320i models from '78 onwards had solid-rotor front
brakes, the '77 320i (and all E21 323?) had the same vented-rotor
front brakes setup. The vented rotor front brakes setup is more
desirable especially in extreme usage (e.g. racing and driver's
schools) because it allows better brakes cooling. You need both the
rotors and the calipers in this upgrade because the vented rotors are
thicker than the solid rotors (the calipers for vented rotors are
also wider spaced to accomodate the thicker rotors).

Here's the required parts ('77 320i) and how much they cost me in
Canadian Dollars (~.73 USD):
$ 5.00 brake caliper (self-serve wrecking yard)
$ 5.00 brake caliper (same)
$ 15.00 brake rotor (self-serve wrecking yard)
$ 15.00 brake rotor (same)
$ 0.00 re-surface rotors (a friend did them)
$ 33.60 brake pads (Repco Deluxe, they DO have an asbestos warning on
package)
$ 30.00 caliper rebuild kits (seals only, for 2 calipers) BMW P/N (?)
34 11 1 116 618

Optional parts:
Tube of grease, suitable for wheel bearings (if you're re-packing the
wheel bearings / hub)
Front hub grease seals
Front wheel bearings
34 11 1 118 711 Anti-squeal shims
34 11 1 150 542 Pin & Spring Kit (brake pad retaining hardware)
34 35 1 180 780 Brake Sensor

Here are the major tools you will need:
17mm socket (wheel studs)
19mm socket (caliper bolts)
11mm 6-sided flare wrench (brake lines)
7mm wrench (caliper bleeder valve)
5mm hex key (rotor retaining screw)
22mm? socket (castellated nut on front hub)
hammer (y'never know)
brake fluid as required, DOT4

I drove my BMW quite enthusiastically for 3 summers and 1 driver's
school and still had plenty of brake pad material and rotor left
(although the rotors were slightly warped).

I recommend you re-use your old hubs if they worked fine up to this
point (also re-pack them with new grease as a minimum). Note that
models through '79 have different (beefier) front hub / spindle /
bearings than '80 onwards but for purposes of this brakes swap the
upgraded rotors and calipers should fit either type of hubs just
fine. I re-used my '82 hubs and re-packed the bearings (might as
well if you're this far in at this point). Also remember to apply
anti-seize on the rotor retaining screw. I drive my Bimmer only in
summer so it's not bad but you all-season folks in the snow belt
oughtta pay heed to this one.

The vented rotors and calipers bolt straight in, you can keep the old
backing plates in place (like I did) or remove them completely for
even more cooling as required (I probably won't remove the backing
plates unless there's some really serious driver's school action
within the next few years). Anybody else out there with vented
rotors that aren't running with backing plates, how about you E21bob,
what's your setup?

I tried (unsuccessfully, whoops it broke) to remove the brake sensor
from the old pads but no big loss. The way I see it, if you do your
own wrenching then you shouldn't need a light to tell you when to
change brake pads. So I re-attached the connector to the wiring
harness and cut off the sensor-end.

To better understand the next few points, think of the rotor as a top
hat with a brim. Upon examination of the vented vs. solid rotors,
they appeared to have very similar or same dimension from inside
friction surface (bottom of brim) to where the wheel fits up (top of
hat). Therefore given this same dimension for both types of rotors
and the thicker 'brim' of the vented rotor, the vented rotor has a
shorter 'top'.

All this leads to why the wheels are now significantly more difficult
to fit on to the hub. My first thoughts on fitting the first wheel:
@#*&$, this wheel should be on by now! The wheel just refused to go
on as easily as before. The grease cap got knocked off, and it was
difficult to line up the stud / hub / wheel. I lined up the
remaining un-installed vented rotor with the inner wheel surface,
clearance should be OK. Before the brakes upgrade, the wheel used to
be able to rest on the 'top of the hat' while you could line up the
studs through wheel into hub and bolt it all together. I found it
much easier to remove the wheel center cap and eyeball the hub
through the wheel while fitting the wheel and then install the wheel
center cap later. The larger calipers also contribute to more
difficult wheel fitment. After completing one side and installing
the wheel, I compared caliper / wheel distance with 'upgraded' vs.
'stock' (quick check using a finger as reference). I could manage to
insert a finger between wheel and caliper on the 'stock' side but the
'upgraded' side had less room. I have the stock cross-spoke alloy
wheels that came with the sport package and assume that the 'fan-jet'
style alloys would have a similar fit (I can verify if you really
really have to know, my mom's car has these wheels).

So by now you are probably expecting driving experiences with the new
setup, I haven't got any! I forgot to stock up on DOT4 brake fluid
so I have yet to fill the reservoir and bleed the brake lines and
calipers. Any better braking would probably be due to different
friction pad material because the rotors and pad area are very
similar (same?). I'm interested to see how little (or how much) the
Repco Deluxes dust the wheels.