There are 2 ways to think it?!? :
i)If you expand your capacity by adding a hdd, you will have limited storage (this means you are restriceted to the capacity of the disks size) but you will have access to your files faster and without changing DVDs or searching to find in wich DVD yo have each file. Also you wont have problems with scratched DVDs and so on.
ii)If you expand your capacity by adding a hdd, you will have practicaly unlimited storage but you !!!MUST!!! use good quality DVDs and use a bag to store them (not the cake-cases,they damage them)
I recomend a DVDRec, you will also be able to write your own DVDs.
Aqua