...As far as noticing a difference - let me explain where the difference comes from.
It's important to understand what memory actually is. People often confuse memory with disk storage, and they are not the same thing. Memory is the space that programs are loaded into in order for the processor to process them. When a program is no longer running, the memory space is again available for loading other programs. Additionally, when a computer is rebooted, the memory is completely purged (as it is when you turn the computer off).
Disk storage is where your programs, documents and settings are permanently stored. It does not purge when you reboot.
OK, now with that out of the way, let's explain why adding memory can sometimes make a difference and sometimes not:
Back in days of old, computers had a fixed amount of memory. When you ran out of memory during an active session, you'd get an 'out of memory' error and you couldn't do anything until you shut down some programs or rebooted.
So, some genius came up with the concept of "virtual memory". Virtual memory is a space allocated on your hard disk that the computer "pretends" is real, physical memory. It's great because you never run out of memory and have to stop working or close some programs, but you have to realize that your hard disk is several orders of magnitude slower than real, physical memory. It's like the difference between going somewhere on a bicycle and flying somewhere on a plane.
This is where the performance issues come into play. If you don't have an adequate amount of physical memory, given the OS needs and the programs you routinely run, your computer will constantly be dipping into virtual memory. It will shuffle certain things in and out of VM, trying desperately to maintain optimal performance.
So, when you add memory to a machine that is using virtual memory on a regular basis, PRESTO, life gets better for your computer and it seems to operate faster. But if you add memory to a computer that already has adequate memory given what you run, you will see no difference in performance. Additionally, your computer becomes no faster than it was the day you removed it from the box, it's just that it's able to operate more efficiently, so in essence, it "feels" faster.
Does that make sense?
