Muckshifter is correct.
If the mother is the only thing ya need I think its ok to use the PC100.
Its not a good idea if ya need a lot of parts and building your future power machine.
If your building a Intell P4 machine you will need 2 sticks of DDR memory, wont work with just 1 stick.
Intell P3 only needs 1, and AMD only needs 1 stick.
In the AMD mother specs if it says 3GB total and it has three stick slots dont start with 512MB. After time that will be a waste of money cause what will ya do will that stick if ya want to fill it 3GB. 3/1 Get a 1GB stick to start if ya dont have the bucks for all 3.
Dont get memorys that will go over the total board specs thats a waste also.
The same with Intell P4 boards if the specs say 4GB and it has four slots get 2 sticks of 1GB to start.
Board specs say 6GB has four stick slots get 1.5GB sticks.
I hope this helps you.
Thats the best way to budget build.