PC2100 runs at a double data rate 133 MHz bus - you will sometimes hear this referred to as a 266 MHz bus.
PC1600 runs at a double data rate 100 MHz bus - also referred to as 200 MHz.
So, in general, if you are getting a 200 MHz FSB Athlon, it's better to get PC1600, if you're getting a 266 MHz FSB Athlon, it's better to get PC2100, that way the processor and memory buses are in synch.
As far as it being faster than RDRAM, that's sort of a technical gray area. RDRAM on the new Pentium 4 systems generally have more bandwidth because they utilize two channels, versus all of the DDR-SDRAM motherboards, which only use one.
For more info, Raystonn could probably give you a much more in depth difference between RDRAM and DDR-SDRAM.
<i>I don't know anything about computers... but I did stay at a Holiday Inn last night...</i> :lol: