First you need to know what the maximum memory supported by the motherboard and bios is. Second, if your Windows OS is 32 bit you will not gain any benefit from the upgrade. 32 bit will only address 3gb with the extra 1gb used for video. In theory 2 sims should always be faster than 1 but if you want to go to 16gb someday you should go with the single 8gb. Your computer would possibly support leaving one of the 2gb in with the 8gb giving you 10gb total.