No need to worry. They operate very differently.
Hard Drive - stores data permanently (so that you can unplug your computer from the wall -- or shut it down).
Memory - stores data in volatile state - It is completely erased when computer loses power. That is why when you lose power to your house, you have to reboot your PC ---- it doesn't resume where you left off because the data is no longer in Memory. But it can read the hard drive, which has the Operating System, stored files, etc.
So with that in mind, the amount of RAM (Memory) you have is only important when you are looking for Performance. If you play video games, or run many applications at once, and are noticing "lag" (pauses, hangs), then you may want to upgrade your RAM. Based on your question, I would recommend having your PC serviced by a professional to have the RAM upgraded, as it can be a difficult task for first timers (need correct ram type, meet motherboard requirements, and know how to install).
Generally, you want to have enough RAM to support your Operating System, with a little extra -- at minimum. Windows XP requires about ~256mb (not even 1gb) to operate. Windows Vista and higher required 2gb as a recommended amount, due to advanced rendering features for the themed window system. It can operate on less (e.g. with Aero disabled), but you will encounter lag - as it is designed for 2gb or higher.
Same "rule of thumb" applies to Mac, or *nix.
Hope this helps.