I have 2133mhz ram and i7-2600k @ 4.5ghz, sad reality is i see no difference compared to my mates 2500k(not overclocked yet) with 1600mhz ram. I'm sure there is marginal gains but not enough that i can tell.
I'd just like to chime in here and say I have been tinkering around with my 2133 kit on my sandy 2600k myself. I'm finding (so far) that my games are responding better especially while running with multi-gpu's with the lower timings @1600 than when my kit is set to it's max cap at those stable timings. It's about how instant the information is transferred with reference to latency than the total bandwidth that really helps graphics overall.
I would go with the 1600 as those are nice timings for a nice price.
GPU is king in gaming, 2500K vs 2600k, 1600 vs 2133 is on playing with the marginals at best.
Except for the fact that this thread is about memory, I would almost completely concur. Though at 2560x1600 I have had to tweak with my north quite an amount to get the fringe and lace on my frames I have wanted. That said, I'm eyeballing a 3rd card early this upcoming year. Ebay special perhaps...
Gpu's do help the most. The op is not making a poor decision at all upgrading to 8gb from 4gb kit though. I saw a greater amount of a performance boost from the amount of ram than I have ever seen on speed upgrades.