We cannot say what the future of RAM will be in 4-5 years, but 1600mhz is standard, and will be for the next few years. Realistically, the difference between 1600, 1866, and 2133 is 1-3 frames per seconds. Nothing wrong with overclocking to 1866 or 2133 as long as the system is stable. You shouldn't spend more money for 1 or 2 extra frames, simply not worth it.
You can get 1600 or 1866 memory for fairly good prices, G Skill sells some competitively priced.
I recommend either Corsair or G. Skill. If you want the cheapest memory price wise that will work well look below:
It won't be todays DRAM - DDR4 will be out shortly, one of the reasons DDR3 is/has been rising pricewise, Yes, can call 1600 standard, but as said, it's entry level...1333 is drying up, about 3-4 months ago there were like 300+ options available at the Egg, now it's about 200.....there were over 460 1600 sets now down to 435 or so and shrinking as they sell out of inventory).
With the release of DDR4 (expected this year) the LOWEST is 1600 and initial specs, per JEDEC run up through 3200 sticks, so yes we can fairly accurately see the direction DRAM is headed, lower voltages and higher freq sticks
Problem I see with DDR4 though too is, initially for the first year or two we will have high prices due to low demand for DDR4. We need DDR4 capable motherboards, which will likely be mainstream with Intel Skylake, hopefully.
A lot of factors in DRAM prices, but your right to call 1600 standard.
I would just hate to see someone spend 20-30 more dollars for 1-3 extra frames in gaming.
May be doing more than gaming, I don't game at all, and most of my clients aren't gamers (maybe occasionally) but I seldom build with under 2133 sticks, and even gamers generally want 2133 or even better, 1866 is about entry on my builds, don't remember the last 1600 set
Most gamers tend to think higher numbers will mean better performance all the time. For gaming computers specifically you can pretty much you can grab any 1600mhz kit and be set due to the difference been so negligible unless your using an integrated GPU solution. If the prices are good though, I see no reason not to get 2133+, but for gamers I'd like to see them invest it elsewhere(GPU for example.)
Most I know go for where ever they can get an increase, seldom will $15 or $20 kick up your GPU, though it will kick up the DRAM....and most also do more with their rigs than game, where faster DRAM can play in significantly
Back on topic, Sefton, if you're not on a tight budget get some good 1866/2133 DDR3 sticks of memory from G.Skill or Corsair like mentioned above, if your on a tight budget, go with the 1600mhz as they are fairly cheap.