I was the first person of anyone I know to have had 2GB in my PC (everyone else had 1GB or less, the difference was like light and day.
Some time later, going from 2GB to 4GB makes another bounding difference, especially if using Fista or W7, and especially if playing modern games.
I am sure there is a difference between 4GB and 8GB, but as others have already pointed out it is of limited use to most people.
With regards to the question of 4GB of fast RAM vs 8GB of normal RAM, if they are at the same rough price then 8GB would be better for some users, specifically those who use multiple programs that eat through memory, whilst 4GB would be better for overclockers, and gamers who usually only run a single app at a time (a game), and have tweaked their systems for their setup anyway.
For the average user there would be nothing to gain by having more than 4GB of RAM, or for that matter very fast RAM, so primarily it comes down to price, at which point cash can be saved (or spent on other components) by using 4GB of normal speed RAM.
Personally I would suggest to anyone that they should get a nice fast SSD, it made a much larger performance difference overall than my last major upgrade did.
i.e. going from a single core AMD 4000+ with 2GB of RAM up to an overclocked C2D with 4GB of RAM made a less noticable performance increase in general use than upgrading to an SSD did (obviously gaming is not counted in that equation).
FYI I currently only have Firefox and task manager open, although NOD32 is running in the background along with Speedfan, VirtualClone Drive, and CCC. I am running W7 64-BIT with 4GB of RAM, here are the numbers according to task manager.
This is with a 64MB Page File (I only have one because some programs seem slightly unstable without one) the above number wouldnt be any different if I had a 2GB page file).
The simple definition of Available vs Free, is that Available is data that is potentially useful that has been "cached" into main memory, but can be overwritten instantly by any program that requires it, unless that programs data is actually within that "cached" portion of RAM.
Free is the amount of RAM that is currently empty.
Available is both combined.
Therefore, my PC currently has 3135MB of RAM that is can use for any program, therefore as has already been pointed out, 32-bit programs generally cant use more than 2GB anyway so 3GB of spare RAM waiting to be used is perfectly fine.
If you have "superfetch" turned on then you need every bit of RAM you can get, as it basically "caches" every program that you have installed before you even run it.
I can really see the "cached" advantage of having heaps of RAM for anyone who uses many programs throughout the day, of for anyone who plays games at the same time. Here is the reason why, if I were to open up BF2 and play a game, no problem, it is still cached from when I played it yesterday, however it eats 2GB of RAM on its own, so therefore it wipes out 2GB of cached programs out of the 3GB that it has already cached, if someone were to regulary open and close many programs that really used RAM, then the more the merrier from the "cached" data perspective, however most of that benefit would be wiped out with the use of an SSD, or as I have done disabling "superfetch".
Main PC, P180, CM Silent Pro 500M, Phenom 840 @ 3.6GHz, 8-GB @ 1,866MHz, 256 GB Samsung 830, 500-GB 7K500, MSI 660Ti Twin Frozr, PC is quieter than my monitor :o
Server, 6-TB RAID-5 array, + 2 x 2-TB backup drives, 380W Enermax Pro82+, 4x very quiet fans, positive pressure only, no exhaust fans
Living Room PC, 3500+, 2-GB RAM, HD501LJ