I've read a lot of references to this, but I searched the forum without finding a clear explanation. It seems if I want bluer, brighter lights, I need to go with 9005 bulbs instead of 9006, and there'* some sort of wiring/relay upgrade to do. Is it that the 9005 bulbs are used in some cars for low and high beams?

no the 9005 light is a high beam only light(65 watts) the 9006 is only 55 watts.
i have not heard of anyone having to upgrade the wiring/relay .

my car lights up the road alot better then it did with the stock 9006 low beam bulb.
here is a site that tells you how to mod the 9005 light to fit in the low beam side.
high beam light in the low beam side, 10 more watts of power and 70% more lumens.
$ cheap too $. i put the cheapest($9) GE 9005 lights in to see if it really worked, it did.
the cheap(9005) GE lights put out a whole bunch more light than the SilverStar 9006
that i had in the car. if i was going to replace the GE'* i would put these in
9005SU Silverstar ULTRA for Ultra Night Vision - The Whitest and Brightest Halogen

http://www.bmwe34.net/E34main/Upgrade/More_light.htm

hope this helps you see at night, i know i do now.
i know i would never go back to the stock (9006) lights.

ROD

Just get the Xenon HID headlight kit

if he wants illegal lights and to have to deal with the lamp monitor constantly telling him his headlights are out that may work