You can probably buy 4x 2Gb DDR2 sticks as 4Gb sticks of DDR2 are very expensive. However consider these factors
1. The RAM will have to be compatible with your motherboard, for that you will need to know the exact make and model of the motherboard (Intel, Gigabyte is not enough information I'm afraid)
2. You will need 4 empty RAM slots on your motherboard. Some motherboards only have 2 RAM slots
3. Check to see if your current RAM is in the form of 1x 2Gb sticks or 2x 1Gb sticks. If it's 1x 2Gb sticks you can buy three more 2Gb sticks but they must be the same specification as the stick you already have (preferably the same make/model) and if you have 2x 1Gb sticks you'll hve to remove them entirely and just buy 4 new sticks
4. If you are running a 32bit operating system it can only use a maximum of 4Gb anyway so 8Gb would be a waste. You can check this by going to the control panel and clicking on "system"