I'm running a similar system to you, a P5X58D-E with an i7 920 D0 mildly overclocked to 3.6Ghz. I was using 6Gb Corsair XMS3 until recently when I upgraded to 16Gb. I looked at it as an investment since the 16Gb DDR3 can be moved to a new system if and when I decide to upgrade motherboard/CPU again. Also - I didn't realise you could run 4x sticks in an X58 based rig with the first three sticks in tri-channel mode and the last in a single channel mode, this is called 'Flex' mode. With that in mind if you do decide to purchase more RAM you would be as well to go for 4x sticks rather than a 3x stick tri-channel specific setup since then at least if you did move away from the X58 based system you would have a dueal channel set of RAM to plop right into your new motherboard immediately. Digging around the price of tri-channel specific ram kits is on a par with or indeed in some cases slightly more expensive than 4 x sticks of dual stuff for some unknown reason. So there's no real additional expense as such.
Check
THIS THREAD I started a couple weeks ago on the subject. As I mentioned, most X58 based systems can run in 'Flex Mode' with 4 sticks of RAM fitted which gives you Triple Channel mode as normal for the first three sticks and the 4th stick runs in a type of single channel mode. There are images in the above thread describing it and what slots the RAM sticks need to go in.
I went for Corsair Vengeance, 4x 4Gb Sticks of CMZ16GX3M4A1600-C9,
THIS STUFF. The only issue I had was that the 3rd and 4th sticks fouled the Coolermaster Hyper 612S heat sink because of the ridiculous heat spreaders on the RAM. I simply removed the heat sinks on the 3rd and 4th sticks carefully as they are not required, especially if you aren't overclocking your memory which I'm not. It really depends on whether you are going to take advantage of the extra RAM, I find it's made no real noticeable difference to gaming save maybe for slightly quicker load times between levels in certain games. But when I use CS6 apps like Photoshop and Fireworks with multiple windows open the extra memory is very noticeable. I also believe that the entire system with 16Gb feels slightly more 'snappy' at opening drives, folders, files and programs than it did with 6Gb.
Lastly, if you do go for a new kit depending on what HSF you have fitted which I imagine will be a half decent one given you are o/c'd to 4.0Ghz, be wary of buying high profile RAM as you may run into issues with it fouling the heat sink like I did.
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