I'll try to clear up a few things.
The native socket 478 chipsets(865/875) only SUPPORT upto 200 FSB. That comes out to DDR400, or PC3200. DDR1 can not be run in a ratio higher than the processor, and the highest FSB processor for the socket 478 is 200 FSB. Now with LGA775 there are processors that are running at 266 FSB, so that is now the highest supported FSB for the processor on those boards(925XE/945*/955x).
Support means that they will guarantee those speeds if you are running the proper hardware. There is no way to run PC4000 on a socket 478 system without overclocking, so why should the company support it? They don't.
That having been said, if you pick out your hardware wisely, you should be able to run PC4000 at its full speed without any problem. I ran an Abit IC7-Max3 with PC4400 at 250 FSB. My northwood was running at 250x15, which was the limiting factor for my RAM, but it wasn't really possible to go any higher on air cooling on my hot room.
Now for the quantity of RAM question. It is a general rule that the more RAM you have in a system, the lower overclock you will have. Having more RAM puts a strain on the northbridge, and you shouldn't expect to break any records while having large quantities of RAM. The i875 supports upto 4GB of RAM at PC3200 speeds.
When you read about really intense overclocking they even shy away from 1gb of RAM because they want the absolute max speed. 256x2 is the lowest that they can get while still benefiting from dual channel memory.
My opinion is that you don't need to run your RAM at anything higher than your processor, and even running it lower shouldn't really be a problem. In benchmarks you will see a difference when running your RAM faster, but you would never feel that difference while doing normal things on your computer, and probably not even during games. If you go from 512mb to 2gb you won't even see a difference in benchmarks if you are running at the same speed. The only time 2gb is better is in real world applications where you are playing with large amounts of data, or maybe many different programs each taking a lot of data.
I am happy with my 1gb, and I didn't see too much of a difference from my 512mb except in games. You might be doing things that take advantage of the 2gb, but I wouldn't recommend the normal using getting that much. Of course I also don't recommend the normal user overclocking at all. You just won't see the benefits from overclocking except in benches, unless if you are overclocked very high. I started to feel my overclock on my northwood when I was getting around 1/5 higher than stock. That was from 3ghz to 3.6ghz. Overclocking is only for fun until you get into pretty high speeds. That's just my opinion though.