Yes, you are correct. Just pick up H4's or H3's respectively for your car in the wattage I specified, and you should be okay (although you could go higher wattage, but that is another story). As I said, the headlight assembly is kinda odd on the W124 E420 and not covered in the manual, so you may want to look into that. I can give you a brief tutorial though, but I was a little afraid to take them off myself the first time, so if you find a service advisor at the dealership, they are more than happy to help you.
As you know, the two intake arms to the engine are connected to black plastic housings that cover your head light assembly. The arms are easy enough to remove, since you will notice the "soft joint" can be bent, and it is easy enough to simply pop the arms off. The tricky part follows; the driver's side intake arm connector has a plug connected to a socket. You have to first remove this plug. Then, you have to apply force UPWARDS and pull the black plastic covers off, same method for both sides. Then, the rest of the head light replacement process is covered in the user's manual. As I said before, I would just go to the dealership; they seem more then happy to replace 'em for me.
Just a suggestion about bulbs. I know there has been a lot of issues regarding the visibility using stock wattage aftermarket bulbs. Nevertheless, I must say I am more inclined myself to get the PIAA Super Plasma's, not so much more visibility, but for look. The stock wattage seem to be fine, and in terms of visibility, I did not detect any noticable difference, other than the light colour output that shines the road. However, a problem with using these bulbs is that they are SO bright, that they make the square head light assembly that house your main high/low beams to appear very DEFINED, which I did not like. So, I think it is in good taste to also replace the fog light assembly with PIAA Super Plasma in H3, which doesn't seem to be carried in North America, but they do have them in Japan, and I can suggest some online places where you can get them from, if you are interested. If you keep your fog lights as well as the head lights on simultaneously, your entire head light housing is completely illuminated, and looks VERY good, with just a little bit of definition between the fog lights and the main high/low beams. Furthermore, the Super Plasma produce a nice blue/purple core that expands/radiates outwards towards a lighter edge that looks very good, and with both fog and high/low beams running, it produces a very beautiful effect. Just my opinion.
The effect is like the picture below, except you will have more of a blue/purple tint to the head lights, and a nicer fading at the transition between the fog light and low/high beam housings (obviously, you don't have the bottom fog lights):
[Edited by DuckMuck on 01-24-2001 at 05:54 AM]