easy upgrade to replace bulb:

http://www.bmwe34.net/e34main/upgrade/more_light.htm

harder, more expensive, although you get HID Xenon lighting:

http://www.bbesound.com/bmw/hidinstall/

I would not recommend:
======================

Altering high-beam bulbs to fit in the low-beam housing. The high-beam
bulb does not have the anti-glare coating on the end so anyone coming at
you WILL be blinded.

HID conversions - the E34 housing really won't work well with one and it
WILL blind people coming at you.

Putting a higher wattage bulb in an E34. The harness and socket are
"just enough" for the standard wattage bulbs. Putting higher current
draw though these components has a high probability of causing them to
fail. That's a bad thing.

What I would recommend:
=======================

If your E34 has H1, H7 or H4 bulbs - visit http://www.powerbulbs.com -
order a set of Osram Silverstar bulbs. These are NOT the same as the
Sylvania bulb of the same name sold in the US. The Sylvania bulb is
inferior to a standard Sylvania bulb in terms of light output. The euro
Osram does put out ~50% MORE light than a standard bulb - but draws the
same wattage. I've ordered from them several times and delivery is
typically a matter of days - and free.

If your E34 uses 9006 and 9005 bulbs - Philips has a 30%+ bulb, or you
can go to www.finemotoring.com and get HIR bulbs - which can give you up
to double the amount of light of your current bulbs.

Also make sure your headlight lenses aren't badly pitted. The pitting
will cost you light, and cause glare for oncoming traffic. If they are
pitted consider putting "StoneGuard" on them - it has the nice property
of index-matching the index of refraction of glass quite well -
effectively making the pits pretty much disappear, and preventing new
ones from forming.

HTH.