The overall MEMORY includes RAM and VIRTUAL memory
so the system treats RAM as part of Virtual memory(disk paging file)
After adding RAM you should check and adjust if necessary the
Virtual memory settings of your pc.
for xp.....
Control Panel
Performance
System
Advanced
performance
Advanced
sometimes with low memory these settings get tweeked
generally speaking the system uses a paging file 1.5x the RAM installed
sometimes people set the upper and lower values the same to FIX the size
of the paging file(preventing growing and shrinking) this makes the system slightly
quicker and stops defragmentation of the paging file. This may have a negative
effect if the RAM is now larger than the paging file.
some applications LOOK at the RAM(or maybe the VIRTUAL memory) size
and decide how much to Chew off so to speak. So just increasing your RAM
just gives em more to chew(sometimes slowing them down).
In other words those apps may have been respectful and not taken too much before
but now they are hungry and want some extra.
most graphics cards share system memory, and RAM can be allocated
to graphics chips in the BIOS. I just mention this as you may want to know that.
Graphics cards cannot grab memory, you have to give it to them.
anyway hope that helps.