You are not quite right. 99% of todays motherboards do not accept the "old" EDO ram.
But, if you want to buy a 100 MHz board, you need the corresponding ram...
To confuse you even more, todays boards support (normally) the 66 and the 100 MHz CPUs. If you fit the 66 MHz rams with a 66 MHz based CPU and you want to upgrade to lets say a 500 MHz CPU, you will need to replace the rams as well.....
In other words, board speed 100 MHz or greater, rams of 100 MHz speed and you are set, at least til next xmas.....