search the forum theres a topic on it
as for the ones available / better
it depends on the system u have. ..
or more specifically the motherboard
now the standard is DDR SD RAM ...
it comes in variations of frequency 266,333,400 MHz
its backward compatible so u can use 266 in a mobo supporting 400 MHz as well
ofcourse the higher the rating the better
the new standard will be DDR2 SD RAM .. but it'll take a while to take off
older standards include
RD RAM,SD RAM,D RAM ... ETC.