Done already at 1600. No visible difference. At 1333, why would anyone do that?
There could be a difference however with the CL parameter. That measures the memory latency. The meory I use are CL8 or CL9 in case of the 3960X. You can find DDR3 that has been tested and selected at CL7 or even lower at high speed but they are horribly expensive. Normally if you have top quality DDR3 from a reputable supplier you can run it one notch below, But you must also take care with the other RAM parameters if you want more and then you have to play with voltages etc. etc. But when you run your memory at the limit you are sure that on a machine that runs 24/7 you will have at least one freeze over a few days aor a week period. I have settings that allow stable operation over weeks and months. My testing is made on that basis.

Out of interest would I gain a over all performance increase going from Kingston 1333 CL9 ram to MUSHKIN REDLINE RIDGEBACK 1600 CL7?

Yes, but the increase would be very small, possibly unnoticeable, and thus not worth it.
Unless you play a specific game or use a specialist application that is known to see improvements with the upgrade there is no point in buying. Save the money for a real upgrade, like a GPU.

mikey159b, you might need to upgrade the Bios, but this is a common situation when Windows allocates system memory to a GPU. If you are using an NVidia GPU you can check by doing the following; Right click on the desktop, click NVidia Control Panel, system information (bottom left corner), and look for 'shared system memory'. I think you can check using Catalyst, should you have an AMD GPU. XP does not 'normally' allocate system RAM to the GPU this way.
Another way to test this in Windows (only really applies to Vista/7) is to click start and type dxdiag. Under display it will tell you how much overall memory is being associated with the GPU. If you know how much is on-board the GPU you can work out what is being allocated from system memory and thus work out what system memory is unavailable to the operating system for other things.

I found it!!! Thank you very much it is in the dxdiag, I have Win7, I have 2 gpu's each with 1 gb of memory of onboard memory, but each is taking 3 gb of memory! That makes the numbers work out, 16gb of total memory minus 2 for each card makes 12, which is what it says!! This is on a 1090T cpu but my Bulldozer FX-6100 cpu does NOT show that, in fact it shows LESS memory than what the card actually has!!
They are both crunching just fine so all is okay, both machines have dual AMD 5770 gpu's in them.
THANK YOU!!!

Many boards are now rated with 1600 Mhz or over and 2000 MHz DDR3 is cheap enough as long as you do not request very low latencies, I mean under CL8. So using 2000 Mhz DDR3 running at 1600 is perfect and safe without stress for the whole system. In that case going down to 1333 would be an overkill.