My personal home computer has 8GB of RAM and that is enough for the things that I do. At work, my desktop has 16GB because I run VM's on it for testing and experimenting. I've never wanted more than that on a desktop or had a need. But I also don't run a lot of these 3d modeling apps and Adobe products either.
An example workflow with Adobe After Effects. I'm working on a 2 minute video. I do a ram preview to see what it's looking like. It renders the whole video in RAM at full size (or whatever size and res full/half/quarter). I watch the video a couple of times (ram is fast so I can play it back at full 30fps) and I see two immediate issues I want to go in and fix. I stop the ram preview and there is a green bar showing which part of the video is stored in the RAM. Say I render the whole 2 minutes to ram, I'd a have a green bar running along the whole 2 minutes. So, I dive into the timeline and go work on the video at the 1:30 to 1:45 segment, the green bar and the content is purged from the RAM cache for that 1:30 to 1:45 segment. I need ram to work, maybe jump into Photoshop and/or Illustrator for a second and then jump back into After Effects. Now want to see my new work, I only have to re-render the 1:30 to 1:45 segment if 0-1:30 and 1:45 to 2:00 was able to stay in RAM.
Also, I can stick unrendered comps between After Effects and Premiere and in my experience, that gobbles up some ram.
So, I don't know when I'm going to need the RAM, but I bet sometime between now and when I decommission this computer, I will be glad I made this decision now.
4 passes into IntelBurn Test on Maximum. Ram is 99% filled up. Temps are under 80c and things look pretty stable.
EDIT: Just dedicated 1GB to my Intel HD4000 in UEFI. I can run my TV off the integrated while I work on my other 2 screens off my video card. The missing 1GB is pretty negligible.