Firstly, to clear one thing up, memory and hard drive space are not the same thing, you are asking about hard drive space, not memory.
No, the computer won't automatically put things onto the D drive when C gets full, you have to put stuff there yourself, you can do that when you download anything by simply selected the D drive instead of the C drive when prompted.
to try and make sure the D drive is used though I'd suggest moving your my documents folder onto the D drive, then anything you save in your docs folder will be on the D driver automatically and it make life a lot easier for future system recovery or backups.
http://www.thunderclo...-avenue/my-documents/