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I have a Mac with 2.66 Ghz core 2 Duo and 8 Gb of Ram. How much difference will I see compared to my 5D MKII files
It depends on how much of your system's resources are used right now processing 5D2 files. A lot of people here are saying "just upgrade RAM". That only makes sense if RAM is a bottleneck. In many cases, it is not. For instance, I am processing thousands of D800 files in Bridge (same applies to Lightroom). If you already have 8 GB of RAM, adding more of it won't really help you much. Bridge and LR effectively only load up one image at a time, so their RAM requirements are much lighter than, say, Photoshop. I have 16GB installed on my system, and while I'm powering my way through the raw processing step, the system never goes much beyond 6 GB of RAM. That's with Chrome, OpenOffice, etc. running along with whatever background processes are there (Google Drive, Explorer, KeePass, etc.)
On the other hand, if you do a lot of work in Photoshop, open up several images at once, use a lot of layers, etc. then adding more RAM could be a benefit to you. Basically, figure out where the bottlenecks are (or will be) in your system, and address those in priority order. If you're using Adobe products, bumping up your GPU won't do anything. You might need more disk space before anything else. Or perhaps just faster disk (e.g., I upgraded to SSDs for my working files). Perhaps your CPU is maxed out because that shiny new SSD is able to shovel file data at it faster than the CPU can process it. There is no one universal answer to your question.