Still using 6Gb on my X58 systems, never ran out of memory once.
I'm in a similar position with an 8GB X79 system which I always planned to upgrade when I needed it but so far I haven't seen it run out of memory during Vegas encodes, blu-ray encodes, Lightroom/PS work or gaming. It's a bit odd now the GPU has as much memory as the entire system but the ram isn't that cheap (around £60 to change to 16GB) and it doesn't seem to be suffering at all, still not showing it running out of memory and performance is where it should be.
John