I like the first response there best. It's why I bought 8GB RAM when I could, even though 4GB at the time was still quite a bit. I haven't ever had to worry about RAM. It will give my PC a longer life when I relegate it to a secondary role too. Had I waited until then to increase RAM, guess what, DDR2 is a lot more expensive. For those reasons, the extra ~$45 was well worth it. It's one thing to go and grab 24GB-32GB RAM for hundreds extra, but an extra ~$50 to double it is when I'll jump on it.
For that reason, I'd be grabbing 16GB if I was buying today, 8GB minimum. It just surprises me that 8GB is ~$50 and people are going with 4GB for these new Sandy Bridge builds.
Granted, this way of doing it isn't for everyone. If 4GB is still arguably not only enough, but plenty, and 8GB is overkill, 16GB may seem ludicrous to some, so I'm not saying "only" 4GB or 8GB is something I don't understand. Having been through even a small share time with PCs as they've grown, and being on both ends of the scale, I say having overkill for a small amount extra is way, way, way better than just paying enough and having too little at some point (even if in the future). That PC will last a lot longer with more RAM (RAM is the one thing that matters most on secondary/older PCs for most uses), and if you're someone who sells stuff off, the extra RAM can sometimes get some of the cost back and/or help it sell quicker.