Warning: completely personal, non scientific opinion here:
The general rules says that every CPU core should have 1 or 2 GB of RAM. 1GB/core is usually plenty for “1 app at a time”, but 2GB/core if you really just want to splurge a bit. So… you’re talking about a quad core CPU that can run 8 threads. Not as good as 8 real cores, but better than most quad core processors too.
Therefore, 8GB RAM is already near the very top of what your computer is going to use. 16GB … I would venture a 1-2% speed increase, with lots of applications open simultaneously. During some intense render jobs you may also get some added stability, but it may be too small to perceive.
I had a “real” 8 core workstation, and the boost from 8GB to 16GB RAM was marginal for most video editing tasks. So…you can imagine an “8 thread” cpu would use the extra RAM even less.
Premiere Pro CS5.5 is going to use your RAM the most – but mostly only during outputs. Avid MC6 won’t care all too much from 8GB unless you’ve got super fancy layers. FCP7 won’t care hardly at all. FCPX…I don’t know, I don’t use it enough.