exactly what they are saying.
It doesn't matter how much RAM you've got, overall performance is based on the bandwith. In order to get a couch through a door it doesn't matter if you've got 2 doors if the couch in 40' wide and the doors are both 20'. What you need is a single 41' door, which allows you to breeze through with room to spare.
Same with RAM, in theory the 533Mhz FSB has like a 4.2GB transfer limit, and PC1066 Rambus has the same. PC800 Rambus is perfect for 400MHZ FSB processors (I think) with like a 3.2GB transfer rate.
PC2700 DDR has a 2.7GB transfer rate.