from what i've been told by various people....

the ram you get also depends on the motherboard you have (thats the thing that all the bits get mounted on)

there are different speeds of the ram too. you have to make sure that all the ram sticks have the same speed (like all 800mhz, no mix and matching a 800 with a 1333)
higher speed doesn't always mean it goes faster, it depends whether or not the mother board's native ram speed matches the ram or not
so if your motherboard was built for 800mhz and you stick in a 1333mhz then it will probably go slower than if you stuck in a 800 or the performance wouldn't really be any difference, because some time has to be spent converting numbers

The latest type of ram is DDR3 (i dunno what that stands for), so yeah... modern motherboards will be able to handle it.
i got my motherboard about 2 months ago last year and it only handles ddr2, so yeah, i think ddr3 is new (well... old now... it's been a few months i think)



the moral of the story is... check out the specs of your motherboard