Well, Gertrude, I disagree.
Adding more RAM is always a cheap way of speeding up your PC because, simply, more ram means that less virtual memory will be used and as chip memory is faster and has faster access times than a harddisk has the overall PC speed increases..
Therefore, more ram = noticible increase in speed, whether you use "all" your ram or not!
Not necessarily the case. When you run out of DIMM slots on the motherboard it's not exactly cheap to replace the motherboard or swap out the DIMM sizes for greater capacities.
Also, more RAM does NOT always mean that less virtual memory is used. A swapfile cannot easily be eradicated if you want to retain dump logs, and 32bit programs are limited themselves to a process size of 2Gb anyway.