Front-side bus: it's the speed at which your CPU can access the computer's RAM. It's fairly important for your overall system performance. But:
1) A really fast FSB is useless if your memory can't run that fast. Really fast RAM can get really expensive, so make sure you check that out.
2) RAM speed, just like CPU speed, can't be described by just a single number. 800mhz RAM could be way more or way less than twice as fast as 400mhz RAM, depending on various things.