I am planning on having a new computer built for me (possibly by Maingear) with the same hardware that you already have, except I was going to get mine with 12 GB of RAM (1333 MHz!). I have been wondering if that will be enough or is overkill, or not enough and I will also need a SSD for CS5 scratch.
It would be of high interest to me if you could let me know what your efficiency results are along with some information about what size files you process, what kind of processing you do, and where, if any your computer bogs down.
To be more specific, in my case I work mostly with 350 MB scans of 6x7 film, sometimes stitching two or three together for focus blends or panoramas. I also have started experimenting (i.e. rented) a Canon 5D II and I am started to do exposure blends, focus blends, and panos which also stresses my current set-up.
I found that NewEgg sells a 12GB kit of 3 Kingston memory cards, DDR3 triple channel at 1333MHZ with the same specs as the 6GB memory in my current computer, for $270. I got the part number from a tech at Kingston (KVR1333D3N9K3/12G,) and it has the same 1.5 volt spec as the memory I'm currently using. My computer is a Dell/Alienware Area 51, and also came with a pair of nVidia GTX 460 video cards (1 GB each) in SLI configuration. I didn't request that, but this was a replacement computer for one that I had real problems with for over 2 years, and Dell was very nice about this. The 2 video cards are unnecessary for photo processing but I admit that I like to fly the Microsoft flight sim, which I can do now with all the graphic features maxed out and still get great frame rates.
Your files are much much bigger than mine. I use an ancient Canon 1dMkii, and the converted RAW files are about 46mb. Usually I end up with5 or more layers, greatly increasing the size. I also do HDR's as well as thee occasional panorama, with file sizes reaching into 150-300MB. I checked the efficiency reading working with my basic files with several adjustment layers, and it didn't drop below 100%. I have6GB RAM and have let Photoshop allocate 71% of it. I have a 1TB Caviar Black drive with a backup of my pictures, and I use that as a Photoshop scratch disk.
I am very pleased with the performance of this computer with Photoshop, although some things still seem to take a lot of time: Merging 5 images to HDR, using Focus Magic (a deconvolution program) for capture sharpening, etc. On the other hand, printing from Qimage, the files almost fly from the computer to the printer (an HP Z3100) compared to anything I have used in the past.
The little boy in me is nagging me to spring for the 12GB memory kit, and if I do give in, it will be for the 1333MHz rather than the 1600MHz, thanks to many of the responses in this thread.
Sorry for being so longwinded.