Unless the Computer Village Elders, like Milo etc. advise otherwise, whilst fitting a new hard drive I would upgrade the RAM to the maximum it will take, if it is not already there.
www.crucial.com have always worked for me, the take a look in your computer and tell 1. What you have, 2.Maximum you can fit 3. Suggested packages, depending on what is already fitted, slots available etc. Have not heard of any security isues with Crucial.