Imo RAM is one of the least things people need to worry about when it comes to performance. As long as you get a fair amount of memory, at least 6GB in your gaming rig, and choose one with a good CAS latency at anything 1333mhz or above, you are set.
People blow money buying memory that has is a kick faster at 1800mhz or whatnot. No problem if the price is fine, but if you are paying more or getting less memory for a faster speed you've forgeted up.
Thorough benchmarking tests show that the difference, even between RAM modules of 1333mhz vs 2000mhz, is so minor you'll never recognize it in real world performance.
Many of the more expensive RAM sticks by reliable companies, like G.Skill or Corsair, will offer this 'gaming ram' which is essentially just RAM packaged in a fancy case with heat pipes, charge people more for what isn't really more in terms of the user experience. Its targeted at the overclocking crowd who will buy into that.
So I don't think anyone should sweat it over buying one RAM stick or two. It might be a little slower, I'm not sure how much by, but I don't think its that important. Who would leave one space open for later upgrade? I'm not sure, someone who intends to buy more RAM within the year but can't afford it now perhaps.
As long as your RAM is of a good brand, and preferably has some kind of heat-dissipating casing, it should be fine. Companies like Corsair don't just pander to overclockers-- they offer products like the XMS3 series, which are aimed at high performance computers over brown town gamers, but only in terms of marketing. It'll still do you just as well as any other RAM sold for 'high-performance'. Thats why I chose it.
In short, RAM is RAM. Its gotten so fast relative to our other technologies that you need to focus more on quality and reliability over performance these days.