The amount of RAM you have, asides from maxing it out for no good reason, is after all up to the needs of the applications you run.
I've got a dual boot machine with 8GB of RAM; when I run Linux for my day-to-day surfing, adminning, etc then most of that memory is completely wasted (except *cough* possibly partially used by firefox...).
That is of course until I have to start 2-3 virtualbox instances running e.g. Win XP/Win 7/whatev to try something I've built out in different versions of web browsers.
Then there's many other workloads to consider... I boot in to windows natively whenever I need to run pshop cs5 which generally eats RAM like hotcakes...