If I have 4 GB ram already installed and I buy this 8gb will it give me 12 gigs total
With 2 expansion slots do they have to be the same size?
Most MB's it should work with(but not always), but realize you will get better performance if both sticks are the same, because it will use what is called Interleave Mode on the ram and operate faster.
I usually try to avoid mismatching ram, also ram speed is important that they are both the same speed. The motherboard tries to auto detect the speed of the ram, 2 different types of ram that are different speeds might confuse it. Be prepared to buy a second stick.