The price of RAM has been really cheap lately. I bought a 12GB kit just for fun and added it to my previous 6GB. With just 6GB installed, I was nearing 90-95% at times, so there's really no reason to not get more RAM, especially at the current price. I would recommend getting more RAM, unless you're looking at the really high end stuff that's still super expensive.
hehe... Seriously though, I've never maxed out 6GB, even when modeling in Autocad and generating CNC toolpaths simultaneously. Not really sure what you guys need 12+ GB for? Nothing wrong with having it, I am just curious what kind of stuff requires that much ram?
I'm personally running 6x2 right now and strongly considering and upgrade to 6x4 (12GB->24GB). 12GB just doesn't seem to cut it for what I'm doing right now (especially with Firefox using over 2GB of RAM on its own, not to mention everything else that I've got running).
If you're like me and sitting at 80-90% RAM usage in your normal day to day stuff, I'd say go for it.
More RAM never hurts, and it's cheap to do. I started out with 8GB with Windows 7 but a lot of my clients claim a noticeable speed increase going from 4GB to 8GB.
Back when I built my system people thought I was crazy going with 8GB. Now most people have upgraded to 8GB, it just costs them double what I paid at the time. If I was building now, I would definitely do 16GB. (Or 12 with triple channel)