GPU [video] is a self contained processor
as is the CPU

they both access the RAM at various times, but it is the same block of RAM
you don't get GPU RAM and CPU RAM

Try and buy a single stick of 512MB RAM that is comp[atible with the RAM you have now

OR

get two MATCHED sticks of 256MB
it really doesn't make that much difference to performance,the single or double path, BUT, it does make a difference ...[ you shouldn't mix 'n' match the RAM sticks, use them as matched pairs or a single]

use your existing RAM as the start of your spare part box