Once you have a HDD you will hardly ever use ram. I have not bought any extra apart from the 3 that came free with my hs2 over 18 months ago. I only use them if I want to temporarily store programs in a series off the hard drive until I have all the episodes to transfer in one operation to a -r. Indeed I don't even leave one in the machine due to the extra time it then takes to start up, merely inserting one to transfer to and from as required.
In effect a hdd works exactly the same as a ram disc, but you can store 8 times as much in the hs2, and 16 times as much in the E100 at any particular recording quality.