Just curious, if gaming is not the primary function of your rig, what may I ask do you do? And what games do you play at what settings?
I concur with LatiosXT on all counts. I might even push the matter regarding your video card. Two 27-inch monitors eh? What resolutions are you running?
I do believe getting another GTX 560 Ti will allow you to SLI and use that nVidia surround thingy. If you are interested in that, that is. Otherwise, your rig could use a new GPU with more muscle (again, depending on what you want to do).
I'm guessing RAM will do very little, if anything, for your system regarding performance.
Another SSD might make sense now, but perhaps the price of 512 GB drives may come down so it would make more sense to replace your 256 GB model sooner or later. Then again, I did read that the price of memory may fluctuate soon.
The monitors are big and inexpensive at the cost of resolution; they run at 1920x1200. I do a fair amount of image editing in Photoshop, and a bit of video encoding. I do a lot of remote access network administration also.
As far as muscle, the 560Ti seems to run Portal 2 just fine for me. I'm not playing a lot else at the moment, but I tend to gravitate toward first person shooters and Deathmatching.
I would think that two 256 GB drives in RAID 0 would give me more throughput than a single 512.
I should mention that my machine is also my primary audio source, hence the Xonar Essence and the Grado cans. I love the iFi speakers, but they seem to be slowly dying, and keeping them alive has proved costly. I may need to go with a proper receiver and a decent set of book shelf speakers and a subwoofer.