All of my machines have only 2GB of RAM. On the Linux boxen, I have absolutely no trouble running firefox with 50 tabs, OpenOffice and Gimp at the same time. I don't monitor FF memory usage, because there isn't any significant problem.
On the other hand on the MacBook I must restart FF regularly, else over 700MB of memory the application gradually slows down and "beachballs" regularly. I don't know what's wrong exactly, because I have all the same extensions.
Out of curiosity, why? You can get 16GB of DDR3 for under $100 these days. The upgrade from 4GB > 16GB and an SSD were huge productivity boosters for me. When you stop worrying about memory management (from a consumer perspective), things are so much snappier.
As I said, on my linux boxen I have no problem with RAM. They're fast and responsive. I have machines with up to 32 GB at hand (and dual octocore CPUs) but really, for a dev workstation a dual core with 2 gigs and 2 24" screens is more than enough to me, I never waited for gvim to open or anything.
OTOH my Mac with 2 GB feels slower everyday. Frankly, I simply decided that my next laptop will be a linux PC instead of a Mac, now that there are credible PC alternatives like the Asus Zenbook Prime (a few years ago Mac laptops were so far ahead it wasn't even worth trying).
Actually I built these machines with parts I had lying around. As I only had 1 GB RAM sticks (and 2 RAM slots), well 2 GB I have. But as I said, I have quite fat systems and for what I need (text editor, xterms, firefox, a little LibreOffice and GIMP from time to time) it doesn't make any meaningful difference.
A few months back I had a 2004 vintage machine with 1GB of RAM and the main annoyance was that firefox was quite long to start up in the morning, and that's about it. Ah, and spamassassin was a tad slow, too, meaning that my emails needed a few minutes more to fetch in the background, no big deal. I switched because the graphic card failed and I hadn't any AGP card anymore :)
To be fair, on modern systems, memory doesn't just "sit there being unused", especially if you keep the system running for a while. Memory not used by running processes will be filled with cached files from disk. Adding additional memory can result in significant improvements, depending on your workload and other hardware.
Whether it's worth the money is a personal decision, of course. It sounds like the GP doesn't use an IDE for development; I've routinely filled up 2GB even on relatively small projects just with Eclipse.
On the other hand on the MacBook I must restart FF regularly, else over 700MB of memory the application gradually slows down and "beachballs" regularly. I don't know what's wrong exactly, because I have all the same extensions.