While it should work sticking just a gig or two more into the computer, there are some things to consider.
The new RAM sticks should preferrably be of the same brand, type and size as the old ones. It SHOULD not matter, but experience says that it does matter. RAM is the main contributor to strange bugs.
- the best thing to do is to get socalled "matched" ram-stics. These are multiple sticks packed together, and these sticks will be as identical as possible making it easier for the memory controller.
Sometimes theres even a difference in seemingly identical sticks, where some older ones are "dual sided" and newer ones are "singe sided" (the black memory banks are on both, respectily one, of the sides of the ram stick). Mixing single/dual sided sticks can confuse the memory controller and lead to strange behaviour. Or work just fine.
You also want to utilize "dual channel" or even "tripple channel" features on your motherboard. If you have the same ammount of identical sticks in the two(three) differently colored memory channels, dual/tripple channel should be enabled and you will experience up to 2x/3x RAM speed. That is why I am going from 6 to 12GB. I already have 3x2GB chips, so the only way to upgrade while still keeping my "tripple channel" on, is to get 3 more 2GB sticks (6x2 total, 2 in each of the three channels on my i7 mobo). This is probably why you were told what you were told, Mad_Fred. That, and the computer store might have wanted to sell stuff they had, instead of ordering up stuff they didnt have with a loss of profit (been there, done that).
http://en.wikipedia.org/wiki/Dual-channel_architecture
http://en.wikipedia.org/wiki/Triple-channel_architecture