What you are reading as your 'D:/' drive, for all intents and purposes, is a Hard Drive or disk. It could be a completely separate hard disk to your drive C:/? Or they could both be part of the same physical drive that is merely partitioned? (Like partitioning a room to make 2 smaller rooms)
The 'Stick of RAM' you mentioned is a completely different item all together. RAM or memory is non permanent space your PC uses whilst switched on.
Your Hard disks (C,D) are permanent storage that retain data even when your PC is switched off.
Now...........back to the problem.
Your 'D:/' drive is 80% full? for a 6 month old machine this sounds a little extreme but the situation all depends on:
A) the original size of your 'D:/' drive
b) the amount of data you are actually storing on it.
Personally, I would look at this drive and remove any unwanted files first of all, before you consider potential upgrades.
Double click 'My Computer'
Double click D:/
You can now see the contents of this drive.
Redders, April 2008