hi im upgrading my memory on my computer and currantly have 1gb (2 x 512mb) ddr2
i have been recommended to purchase corsair twin 2x6400c4d 2048mb ddr2 (2x1gb)

so my question is even though i have spare slots for the new memory do i have to remove the old memory completely and replace with the new
or just add the new with the old?????
thanks

You need to have matched specs to run in dual channel without possible confilicts, if the specs are the same between the 512MB modules and the 1GB modules they will work in dual channel but will only run at the speed of the slower modules. I would just go with the two one gig modules.

You shouldn't have to take out the old sticks, if you have opened slots.

If the specs match those of the new modules that is true, but the new modules will underclock to the speed of the slower modules. He will be better off just running the two new modules.

thanks for the advice