| posts: 4,450 | Location: Australia
DDR3-1600 8GB definitely as a minimum. DDR3-2133 can give some benefits depending on what you are doing, it all depends on how the applications use the memory. Most games don't have high loads on the memory (and they're still almost all 32-bit), but this may change in the near future. Games ported from (or to) the new Xbox for example are expected to have a much higher memory footprint, so games may start to see a couple of FPS benefit from the faster memory.
It's like the i7 hyperthreading argument. If a game can't make use of the extra 4 simultaneous threads on offer on an i7-3770k etc, then hyperthreading won't give any performance benefit. Almost all games are like this, apart from what was it? Crysis 3?, but that is an indication of what you can expect in the future.
It all comes down to price. Only an idiot would buy DDR3-1333 now (sorry to all the idiots out there), since the price is basically identical to DDR3-1600. DDR3-2133 can be bought for around $10 more, which I think makes it worthwhile.