Also here I would certainly check the website as not all computers accept a mix of RAM capacity, some have twin slots that require similar capacity in each slot (like HP for example) so then you can't put 1 GB extra in as one memorycard, but have to add 2 x 512 MB...., and there are loads more options. Fortunatelly however, PC manufacturers tend to leave off this limitation. Honoustly, I ship loads of parts for Dell, but I have no clue what RAM settings they use. Recently upgraded mine (home built) and my wife's (HP) and found these ready made PC's can be somewhat tricky.
Who said computers make life simple ??
But how about just calling your local computer "nerd", he should know, or he would at least be able to figure it out .....