RAM is sooo cheap today compared to what it once was. Well, all things tech related are sooo cheap. (yes, yes, I know, "cheap" is relative).
For your average user/gamer TODAY, 4G of ram is enough. The Rig I built back in 2006 had 4G of ram, it was fine and dandy. I just replaced that with a new rig I built back in May. I have 8G in there. Why? Cause RAM is freaking cheap.
Do I use all 8G? Never. Not even close. However, I think that's going to change. Here is my reasoning.
Prior to windows 7, if you wanted to take advantage of a 64bit system, you had to use the 64bit version of vista (or xp). As we all know, vista took a PR beating when it came out, and never really caught on.
This meant, by and large, MOST people ran the 32bit version of windows XP. It's what the average user has installed. That's a fact (go look it up). This has put a cap on system resources. A program running in Windows XP Pro 32bit can access up to 2GB of RAM. That's it.
That changes with the 64bit version of windows 7. Each application (process) can actually handle up to 4TB of memory. (yes, yes, way beyond what technology can do).
Now, what does this mean? Computers built now a days, are all 64 bit. (Do they even make 32bit systems anymore??) It's been like that for awhile. We just didn't have the OS to support it. As more and more people start running a true 64bit platform, software developers will start to go past that 2G limit.
I can see, not to far off, where you will HAVE to have a 64bit system to effectively run a game. Before to long, those games are going to be needing to grab 3 and 4G of RAM. I really don't think that is very far away.