I have a 32GB windows 8 system that runs a 20GB virtual machine. This should leave 12GB for Windows which is more than enough - I have several 8GB windows machines that run fine.
In fact it runs with nearly 2GB free (or 1GB standby and 1GB free), which is more than most windows machines I've used, yet it is forever crawling and swapping. Does windows start panicing about free memory when it has a certain amount left, or a certain %? Because it seems to get into trouble when it is more than 90% memory used, despite the fact that that leaves 2GB free, where an 8GB machine with 2GB free (or 75% usage) seems to run fine.
For comparison, here is a RamMap report from an 8GB system running the same applications, with the same files and webpages open:
The 8GB machine is fine. Still opens applications without blinking, will switch tabs without even thinking about it. The 32GB machine (that actaully has more free in absolute terms, even if that equates to a lower % of the total), thinks for ages, co-incident with a high disk usage, if you switch a tab.