What Windows version are you using? If it's XP/2k don't worry their memory management is ok (or at least better than the old 9x). So don't worry
As for the HDD - the bigger the HDD (GB) the more space you'll have for games
. HDD Cache is the factor that could (not sure to what extent) bring speed to gaming and other PC related activities.
(edit). Just found
~THIS~ article - could be the reason some ppl talk about this problem