I picked up a tip the other day from a computer dude in a small shop, i needed another chunk of memory and i couldnt find any for less than a lottery win.

say your motherboard has 2 slots for memory and for an example you are running pc 2700 512 ddr ram
you can add in to slot 2 only!!! pc 3200 512 ddr ram when you install and reboot check your bios and it should downgrade the pc3200 to pc2700.

does it work yes it does

the correct memory MUST be in slot 1 obviously if you have 4 slots slot 1 + 3 should be the correct memory for your machine

I learn every day :)

Well the memory speed will be related to your front side bus speed

PC1600 DDR200 100MHz
PC2100 DDR266 133MHz
PC2700 DDR333 166MHz
PC3200 DDR400 200MHz

Also a number of modern chipsets support dual channel memory configuration, in which you would use a matched pair of memory modules in certain slots on your mobo. The modules are usually sold together in the same package.