IF you have more RAM it becomes more or less irrelevant what software is "Resifdent" (RUNNING) even if you aren't actually using it...
There is a bunch of stuff sitting in the background.
On my own computer with 2Gb of RAM (all my MB will accept) I can tell you that just sitting and idling "the system" is using ~600Mb of my available memory. That leaves me 1300-odd Mb for whatever I might actually want to do.....
Do remember that a CD only has 702Mb of data (presuming that it's full).
Now understand if your disc drive is frequently accessing the disc
there is something else going on because in theory your drive should spin up and read that data, store it in RAM and play it back from your RAM, not from the physical disc.
Another issue is if you HDD is nearly full or that you simply don't have "contiguous" available space? this will create other problems with the way the system allocates memory
"Defragmenting" your drive isn't only about having fragmented files but how the empty available space and your "Paging file" or "Reserve system space" is on the disc.
A Paging file, swap file or reserve system space that is not in a contiguous block of system space will often not work right
or will sometimes not work at all!
And when that happens the symptoms sound remarkably like what you are complaining of.
More RAM is not a panacea, but does a damned nice job of hiding "other"
problems.... and frankly a problem you don't
know that you have is a problem you wouldn't be trying to fix.
So your saying that your HDD was defragged recently probably means
something different than what I think of when I say "recently"
To me recently means in the last 48hours.