I bought a custom made gaming PC about a year ago & it came with the 6GB of Samsung ram.
Since I bought the PC I have upgraded the 2 GTX 260's that it came with to 2 GTX 560 TI SOC cards from Gigabyte & also added the Coolermaster Hyper 212 plus heatsink & I'm now looking at upgrading the Ram to some good quality parts.
1x,2x,3x,4x will ALL WORK; you lose Dual Channel with odd slots or unequal GB/Channel or ~1% performance hit from broken Dual Channel with DDR3. Samsung isn't cheap RAM, they produce 'Full Spec' OEM and server memory and it's as good as anything.
edit: the builder might have done you a favor if the price of 2x2GB and 3x2GB were closely priced - MORE GB WINS!
Sure the F3-10666CL9D-8GBSR will work, but IF your CPU is faster than 2.66GHz then I would get DDR3-1600 before a more less side grade to G.SKILL any better performance will be solely from the +2GB and nothing else or 0%~2% increase.