raaaaaaaaaaaaaaammmmmm...me

ok ive got a question for you all ....(oh just a little back ground....im in iraq trying to stay on top of things.....so when i come home....im not like how do you turn this thing on).....so back to my question ....is thier any diffrence between ddr ram 1600 21 27 32...i know it changes at 400 and i know ddr 2 is comming out in a lil bit...but what is the diffrece....from what i understand a stick of 512 ddr 3200 will work in a ddr 2100 slot.... if im wrong let me know...oh and one more..........can i buy a stick ddr ram at 1 gig (im not talking 2 sticks of 512 just 1 stick at a 1gig)