I hope the RAM is not 100Hz !!
Seriously, if the RAM is 100MHz, that's probably the speed of the "front-side". If the front side bus is faster, check what the extra cost of faster RAM would be.
If you are going for the "dog's b0ll0cks" processor (well, it was last week!, try to make sure you can feed that hungry bugger all the bytes it can chew!