I've got a P4 2.8 and it's one REALLY REALLY fast chip, you couldn't feed it enough memory bandwidth if you wanted to. Well, I guess you could....
YES, pc2100 ram would definately cause a huge bottleneck, especially since the the bus is quad-pumped to an effective 533mhz before o/cing.
Now it depends on what you want to spend, but the most bandwith would be from PC1066 RDRAM, or some really overclocked DDR.
And as for notebook memory.........I don't know too much about that but NewEgg has some 266mhz DDR SODIMM memory which is pc2100, there could be faster stuff out there........