RAM for MSI motherboards

I would get 1 1gb stick of crucial, corsair, kingston, or samsung pc2100, which is the maximum speed your bios will recognize. It's also possible pc2700 will work, but run at the slower speed. You don't need to worry about single versus dual channel ram speed on such an old board. If you prefer, 2x512 will also work. Avoid high latency ram; it may not even post. Single sided ram is the best quality; this means that all the chips are on one side only of the stick. The memoryx.net ram should work fine, but ask them what brand is listed on the ram label. Most are crucial or micron in my area. Try cpuid for more information about your old ram. It's a free download. I have no personal experience with your board. Good luck.

I can't still understand what happens if I installa an "overspeed" DDR memory on my system, e.g. PC4200 rather than PC1600: will it just run slower, or will it be damaged together with the PC? What does maximum supported speed depend on? Motherboard or BIOS?

No damage. Ddr 184 pin ram is all the same voltage for most speeds, about 2.5 volts if I recall. It won't hurt anything to try the faster ram. If your board is old and it fails, then the faster ram might run in another socket a board. If you have no plans to replace your board with another ddr board after it dies, then use the pc2100. Anything faster than pc2700 might not post. Your board chipset is very old.

It eventually resulted in DDR 400 512 MB benig suitable for all the PCs.

actually DDR 400 is NOT suitable for all PC: some DDR 400 modules work (bought new) , others (second hand) do not, don't know why.
So, this thread has no "best answer".