|Intel systems use a FSB, AMD systems do not...I assume your questions are related to Intel systems.|
I'm not sure what you mean by "motherboard speed" other than FSB? The FSB is the communication path between the CPU & the memory controller which is built into the northbridge chipset on the motherboard. All Intel motherboards support several different FSB speeds. It varies from board to board but the most common speeds are 533MHz, 800MHz, 1066MHz, 1333MHz.
When selecting the CPU, board & RAM, you have to look at the specs of each to make sure all the parts you choose are compatible with each other. For instance, if you choose a CPU that runs at 1066MHz FSB, you'll need a board that supports 1066MHz FSB....you'll also have to make sure the CPU & board are "socket compatible". In other words, if you have a socket 775 CPU, you'll need a socket 775 board. Then you'll need to find out which type/speed of RAM is supported by the motherboard & choose the RAM accordingly. The most common RAM used these days is DDR2. Older boards use DDR but the newest boards use DDR3. Once you determine which type of RAM your board uses, you have to choose the right speed.
Using the same example, let's say you've selected a socket 775 CPU that runs at 1066MHz FSB & a socket 775 motherboard that supports 1066MHz FSB. By looking at the board specs, you'll know that it supports DDR2 so then all you'll need to do is decide on which will work best - DDR2-533, DDR2-667, DDR2-800, DDR2-1066, etc. Currently, DDR2-800 offers the best bang for your buck. Numerous online sites such as newegg.com or frys.com sell 4GB (2 x 2GB) DDR2-800 for about $25 after rebate. You can either run the RAM at it's default speed or manually configure the memory speed settings in the BIOS to provide the best possible performance.