pcxx00 is the type of ram modules that your computer will take,
they are backward compatible though,
for example
if you buy a pc3200 (DDR400) module, then you can use it in a pc2700 mainboard (which supports a maximum of DDR333, however the ram modules, rated at DDR400 (ie 400mhz) will only run at 333mhz (because of the limitation of that mainboard)
i suggest you find out what mainboard you have, before you purchase ram, if it's fairly recent, then it should support pc3200 (ddr400) -- if not, then go for pc2100 (266mhz)
if you're using a pentium4, it'll use rambus/RDRAM, but the p4's these days run on pcxx00, so that will work too.
go to
http://www.cpuid.com/cpuz.php#download
download cpu-z 1.19c
and you'll find out from there
or you could take a look in your device manager (though i doubt it will tell you anything but the manufacturer of the mainboard)
so your best bet will be cpuid.com
download it, run it and go to your 'mainboard tab'
it will tell you things about your processor, your current mainboard and the type of ram installed, so you'll know from there
to answer your question,
the brand/model of your mainboard will determine the type of memory that you're able to run.
e.g. if you have an asus mobo for amd cpu's,
then you'll likely be running pc2100 or 2700 ram (266/333mhz)
but if you are using a new p4
then it'll be any mainboard, (intel, abit, asus, etc.)
and you'll be using pc3200/ddr400
first things first, find out your mainboard
take a look at the online manual for it
and find out what type of ram modules your mainboard can use.
if your computer is REALLY old, then it will run SDRAM, not DDR.