I just run top and sorted by memory usage. I see firefox (with 10 tabs open), thunderbird, emacs, skype, nautilus, dropbox, Xorg followed by many other system applications. I have mysql and postgresql running by the don't make it into the top 50 or something. If I had a ruby on rails application running, or cassandra, they'd be in the top 10. I can run all of these quite comfortably together on my 4 GB laptop, unless something gets unexpectedly too big and I have to kill it. But I'm always a little on swap. I know that if I start Eclipse I have to close something else. IntelliJ is much more RAM friendly but luckily my job doesn't involve stuff for which I need an IDE. emacs is really enough. Even vim would do.
What I can't run in 4 GB is all of the above plus more than one VM. Actually if the VM is not a headless Linux server or a WinXP client I can't run it unless I close some of the applications I keep open all the time. The free Win7 and Win8 VMs from http://www.modern.ie/en-us/virtualization-tools#downloads tend to use a lot of RAM and if one does web development it nice to have a couple of them open for testing on IE. That's less and less important because all of my customers moved to Firefox or Chrome over the years but some of *their* customers are still on IE, which is getting more standard. (BTW, I used to run this laptop with Windows and a virtualized Linux development server, but as my job is to develop for Linux servers and web browsers I decided to go the other way round and I'm perfectly happy with that.)
Furthermore there are occasional memory hogs. A sample program from a Coursera big data course (don't remember which one) required 2 GB of RAM to process some sample csv file for an assignment. Those were program and data provided by the teacher and there was nothing I could do about it. Plus I do occasional video editing. Extra RAM is always handy.
Because of those memory constraints (and end of support) I'm looking for a new laptop and I'm not considering anything that can't be upgraded at 16 GB minimum. I like the internals of HP's new Zbook 15 (fully user serviceable parts, up to 32 GB RAM, up to two disks, etc). I don't like some of its exteriors: the keyboard with small arrow keys and the number pad which turns it into an interface for left handed people (the touchpad is placed to the very left, which is common but inappropriate for 90% of the population). I definitely prefer the keyboard arrangement of their Elitebook 850 g1 (about the same width and no numberpad), which stops at 16 GB and is a weaker machine in many other ways. Actually I prefer the case of my current nc8430, which has a better keyboard (more key travel) and a 16:10 screen (the zbook has to be wider to raise to the same screen height). My Core 2 Duo is still good enough for my job, disk speed (or lack of, compared to SSDs) is not a problem but obviously everybody welcomes a faster machine, me too. However the limited memory is definitely becoming a problem.