there are a few different types of memory.
there is SDram, which is older memory used in old athlon and p3- small P4s called pc100/133
then the most used now is DDR, which runs at different speeds, the most common are pc2100(ddr266), pc2700(ddr333), and pc3200(ddr400) which is the standard now. any higher ddr is overclocking headroom, and wont give you a performance increase with out overclocking.
then there is the newest type of memory, DDR2, which is used in only the newest P4 systems, that has speeds of PC4200 and PC5400
it depends on what kind of system you have, but most likely it uses DDR400 ram
manufactuars dont really matter as long as its the same type of ram