I have a Dell 1545 laptop (nothing great, but serviceable)... it came with only 2gb RAM stock. I want to upgrade that to 4GB... can I just purchase an additional 2gb card and install it in the 2nd card slot (it would obviously be a different brand/manufacturer than the stock)... or do both cards need to be identical in brand - do I need to buy 2 of 2gb cards from the same manufacturer? Thanks in advance!