16GB will not be faster than 8GB in regards to raw RAM performance. In fact, quite the opposite could happen depending on the CPU in question and its revision level (C2 or C3 as of late, C3 being more capable), and the motherboard being used.
Only way 16GB is going to be faster, is if you are actually using up your 8GB doing some major heavy duty computing, in which case you might be choked and using up pagefile space on the HDD instead of RAM, which would be super slow.
With that being said, unless you are doing some serious video editing (production level stuff), or running a web server, or photoshopping for a magazine cover, 8GB is more than enough. Plenty. 4GB is actually sufficient for almost all gaming right now.
In regards to overclocking on an AMD platform, most of the time you will have better results with 2 stick instead of 4, although the C3 revision CPU's seem to be doing much better now when strapped with 4 sticks. This is due to the memory controller being onboard the CPU and henceforth known as the IMC (integrated memory controller). So the holdup in performance was a CPU issue and like I said, has been remedied to a great degree in newer revision CPU's. Revision of your CPU can be viewed using CPU-Z software.
Do yourself a favor, download CPU-Z and MaxxMEM and start familiarizing yourself with the BIOS. Starting out, you'll have much better results with the 8GB. If you find you need additional RAM, or you turn into a RAM junkie and just want the challenge or the bragging rights, buy 8 more GB and attempt to overclock that and make it stable.