Go look at benchmarks, the move from 1333 to 1600 is minimal and percentage based typically boosting frame rates by 3-5%. If your rig is currently capable of putting out 60fps then a 3% increase in frame rate is going to net you a grand total of 1.8 additional frames per second, which is not exactly noticeable. If your system is slower than that then there would be literally no difference.
The bigger question is if upgrading from 4GB to 8GB of ram is really the best use of your money in the first place. When I built my current rig I originally put in 16GB of ram, but I never used anywhere near that, so I put 8GB of it in my wife's system when I rebuilt it last year. Looking at my ram usage right now I am using 3.7GB of my 8 and I have several browser tabs open, plus music, plus office applicaitons, and a game, as well as all of the normal background software (antivirus, steam, hardware utilities, etc.). There are a few games (like Skyrim with mods) which push my system well past that 4GB mark (close to 6.5GB), but honestly very few games do, and if I only had 4GB of ram then I could get by just fine by closing other applications and utilities.
At the end of the day you are talking about $50. I don't know what your total budget looks like, or what hardware you are running, but more likely than not that $50 is probably better spent added to your GPU allotment. $50 can be the difference between a humble HD7770 and a capable HD7850/260x. Or moving from a capable 260x to a respectable 270x. We are talking about 50-100% performance increases, where Ram upgrades are going to give you mere fractions of that kind of benefit.
The other issue is that in a year or so 8GB of ram is simply not going to be enough. Consoles today have 8GB of ram in them and they are still running under 1080p resolution. Desktops running at 1200p or higher, with larger texture packs, more robust AI, and other mods and additions are going to need more than 8GB of ram to play the same games properly. In other words, 4GB if often adequate (even if just barely) for today, but 8GB is simply not going to be enough in the near future.
My advice is to keep your 4GB for now as a stop-gap measure and throw all of the money that you can at a quad core intel or 8 core AMD chip and the biggest, baddest GPU you can afford with the most VRAM possible. Then, in a year or two, when these next gen titles start filtering through consider an upgrade to 16GB of ram (and possibly an even bigger GPU). As you will probably need to replace all of your ram, you can then decide between 16GB of 1333 and 16GB of 1600 memory, and often times (especially during sales and specials) the 1600 memory is actually cheaper than the 1333 stuff.