I'll try, although many here can do a better job ot it. I'll use my personal experience.
I have a motherboard that, according to specs, takes PC3200. However, this board will not run properly if I install all PC3200 modules. It will run without problems if I install PC3200 and PC2100 together...because the motherboard is geared to run at the slowest speed when RAM modules do not match.
Since my PC2100 module is 512MB and my 4 PC3200 modules are each 1GB...I want to run the maximum amount of memory, if possible.
Every motherboard system has a setting that governs the RAM speed, allowing a smooth coupling between the RAM installed and the system. Some BIOSes allow users to access this, some don't. Mine does.
After I had done extensive testing with Memtest to figure all this out, I finally thought that I would see if I could adjust any of the settings for my board/RAM in my BIOS.
For my board, the default speed was 200. The range for adjustment was listed as 100 to 266 (I think). So I decided to see what would happen if I lowered the speed (an under-clocking, as opposed to overclocking), I wanted to know if that would allow 2 modules of PC3200 to operate together without BSODS, strange error messages, inability to access the XP CD properly, etc.
So...I changed the speed to 166, saved the settings in the BIOS, and then rebooted. All my instabilities disappeared, I was much happier
In retrospect, I now kind of believe that the modules may be faulty...but the modules worked fine with other boards at the rated speed. I only know that this adjustment by me is the only thing that allowed those modules to work properly (without problems) with that board.
Every BIOS is different, so there is no standard way of determining the adjustments that a user might make regarding memory...other than exploring the specific BIOS settings available in a system.