The short answer is:
(1) Gamers are constantly playing with the insides of their PCs, so they don't worry about the future, and
(2) Gamers don't understand RAM. (see below)
Wolverine said: "They only recommend 8GB because they don't want you to have 4GB."
Given that most RAM comes in powers of 2, what is there between 4GB and 8GB? I mean, if you are buying more than 4GB, You get at least 8GB, right?
ClucthC said: "Most games are still written in 32 bit code. As a result, they can only access 4GB system memory."
This is a common myth.
http://stackoverflow.com/questions/5916959/can-a-32-bit-program-use-more-than-4gb-of-memory-on-a-64-bit-os
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778%28v=vs.85%29.aspx
JKHoward said: "They could use up to like... a billion GB in 64-bit!
"
Every processor and motherboard has a limit on the RAM it can take.\
Here's an example that takes up to 32GB:
http://ark.intel.com/products/75122/Intel-Core-i7-4770-Processor-8M-Cache-up-to-3_90-GHz
Wolverine96 said: "Yes, I was meaning to start with 1x8GB, and after upgrading later, you will have 2x8GB. In the meantime, you would just have to suffer from not having dual-channel benefits. But I'm pretty sure 2x8GB is better than 4x4GB. I think it's faster, but if not, at least you could upgrade to 32GB later!"
In a dual-channel motherboard, it is important (for performance) to install RAM in pairs. If You have a pair of yellow DIMM sockets, and a pair of green DIMMs, install the first pair of DIMMs in the yellow DIMM sockets. It's always the lighter colored DIMM sockets that you use first.
No, 2 DIMMs will not be faster than 4. Most gaming PCs use cheap motherboards and processor sockets that only take dual channel RAM. In real PCs that use quad channel RAM, it's important (for performance) to install RAM in sets of four.
MrCanEHdian said: "then 16GB will almost be necessary in 5 years I suspect."
Moore's Law says the number of transistors will double every 18 months, not every 5 years.
People who buy twice the PC they currently use need a new PC every 18 months.
People who buy 4x the PC they currently use need a new PC every 3 years.
People who buy 8x the PC they currently use need a new PC every 4.5 years.
I buy a new PC when I can afford a system with 10x the power of my old PC. That means cache, RAM, video memory, etc.
One final note: Buy more than what you need today, and plan to upgrade. You may not actually upgrade. If you upgrade, it can save the cost of a new PC.