Firefox memory weirdness

Dear Interweb, please explain me this:

If I have 80 tabs open in Firefox and I have 2GB of physical memory and I then restart my browser. It eats up 1GB easily and then slowly grows until it hits 1.5GB of memory (almost all available).

If I have the same 80 tabs open in Firefox and I add 2Gb of physical memory (total being 4GB) to exactly the same machine and I then restart that same browser. It eats up only 300MB of memory and never really grows much beyond 400MB.

What am I missing here ?

Tested on a CentOS 5.2 with Firefox 3.0.5.

Laptop memory usage

A week ago I visited MediaMarkt looking for some money to spend (and in the same throw improve my own life). There was this amazing deal to buy 2x 1GB DDR2 laptop-memory for only 77€. Faster than my laptop needs, but it happens to work flawlessly.

I always felt that my laptop could use more memory since I have this bad habit to use Firefox tabs as permanent memory. If I stumble upon interesting information but have limited time to read it, I just open a new tab to continue surfing.