How much do you REALLY need? [RAM]

How much RAM do you really need? I see people out there with like 12GB and 16GB even up to 24GB and I'm sure it goes on and on. But how much do you really need? Do those w/ massive amounts of RAM, do you plan for peak times? Are those like people how do hours worth of 3D rendering and all that fun stuff? And then there's the PC companies (HP, Dell, etc.) even in their mid-range computers they include up to 8GB of RAM, but it's almost always partnered with not so powerful components. Is RAM an easy and cheap way to make a computer look more powerful than it really is? And like 70% of the time it's just DDR2 800Mhz RAM, so it's nothing really that impressive speed wise. IDK, I was just wondering how you could ever use soo much RAM, and why manufacturers can never seem to give you enough even though the rest of the system is of average specs.

well, most companies do that for the wow facter like "Look at this computer, it has 12GB of MEMORY!!!!!"
really for gaming 4GB is plenty
3d rendering, photoshop, as much as you can get

if you think 24GB is a lot, you should check out the supermicro dual LGA1366 motherboard with 18 DDR3 slots, with 3x4GB sets for servers you can hit 72GB with that motherboard