Newb memory question.

Theres no harm in leaving your original 2GB.
If you get 4 x 2GB (here are your 4 identical modules) thats 8GB +2GB Original.
If you're looking to upgrade your RAM I recommened www.MemoryAmerica.com. - last time I checked their 8GB kits were running at $346.

 

We have Guide on Understanding Intel Mac RAM under the Guides tab, Hardware.

The answer to the question depends on how you are using the RAM.

RAM in the 3 and 4 sockets on each riser is slower than RAM in the 1 and 2 sockets (higher latency, because the request goes to the 1 or 2 socket first, and is forwarded by the chip on that FB-DIMM to the 3 r 4 socket)

So if you gain more application performance by having the extra 2 GB total RAM, than you lose through additional latency, then you would have faster performance.

In the USA, check out Data Memory Systems - $356 for 8 GB, excellent reputation and service.

I don't know MemoryAmerica -- their domain has been in existence for 1 year. The WHOIS domain information is stealthed, which I never like.