See, if you want to upgrade to 8 GB, there is more than one way.
All the below configurations will work. Provided you have slots on your motherboard.
1 X 8 GB
2 X 4 GB
4 X 2 GB
8 X 1 GB
But in your case, it seems you already have a 2 GB stick. You can a) add 6 GB and make it a dissimilar capacity. But it may or may not work depending on your motherboard. And even when it does, it may or may not operate at the best frequency.
As a recommendation, you don't want to put dissimilar capacity RAMs in a desktop. See if you can exchange the 2 GB stick with a pair of 4 GB. Otherwise, if you have slots on the motherboard, buy 3 more of the same model that you already have.
Also, frequency and simm/dimm type and ddr# matter as well, so start with your CPU, repeat for motherboard on compatibility on the frequency and type that you are about to purchase. Last but not least, your operating system also needs to be verified if it can access beyond 4 GB.
Long post, but hope it helps.
PS: Now a days, all information is available on the Internet and you can search for it as well.