I'd think how much ram you use really depends on what you're running....
Without any fancy bench marks, but just from my own experience of DAW usages as a home studio hobbyist...with my single quad core Win XP machine, I could run most of my projects fine, with the installed 4 GB ram. But when I really started doing projects using a lot more VSTi's, things started getting bogged down, and strange things happened (like my various audio and midi tracks started going out of sync from each other, etc). So my answer would also be VSTi's hogging up the ram...however, what if I had ran the same amount of VSTi's, but used more of 'something else' instead...maybe that would have made me think that was hogging ram (?).
However...I'm now using 3 Win XP machines together (12 cores of CPU & 12 GB ram combined), each have their own tasks, at any given time.... whether it be acting as a dedicated VSTi machine or as a dedicated audio recording machine, etc.
What I can say at this point - even though I still lean toward VSTi's being the ram hogs - is that when I'm using one of these XP boxes just as a dedicated VSTi 'only' machine (and I'm talking using more VSTi's than what I was using when I was just using a single XP machine for everything) my ram usage on a dedicated VSTi machine is well below the 4 GB I have installed....in fact I'm talking about all resources are being taxed very little.... Same with the audio machine. Go figure!
This all leads me to the notion (no bench marks as I've said) that whatever you're doing on any given DAW system, over-doing it with heavy demands starts to tax it's resources in ways that almost don't seem to add up, when comparing it to two systems that are well under load. It's like a threshold get's reached and your system caves in...much like the idea of when hard drives get close to full, it doesn't want to work well anymore...even though you're not at full capacity.
