PC2100 is the technical name for DDR266Mhz. That is, PC2100=DDR266Mhz.
Faster DDR memorys are out, like DDR333-PC2700 and DDR400-PC3100 but in case your mother board accepts them (take a look at the manual) don\'t buy them if you have are gonna keep DDR 266 modules, because they will run at the speed of the slowest module.
On the other hand, if the speed of your proccesor front bus is 266Mhz, all the info must go through it, so buying faster RAM won\'t help too much either.
I would say stick with the 266DDR until you upgrade your motherboard and proccesor [img]images/icons/wink.gif[/img]
SDRAM is a kind of RAM used before, very cheap, but it only can go at 133Mhz. Some motherboards still mount slots for them, but they\'re dissapearing fast. [img]images/icons/smile.gif[/img]
RAMBUS is the RAM used in some P4, very fast(800Mhz) but also very expensive. That\'s why some motherboards for P4 now feature DDR slots, is a better compromise price/quality
Pfiu, that was longer than I expected [img]images/icons/grin.gif[/img]