The hard drive is your computers storage and it's where all your files are kept when they are not being used, the contents of the hard drive are kept when the computer is off but hard drives are fairly slow, so the larger the hard drive the more information you can store (be that pictures, videos, documents, music.... whatever)
The RAM is what your computer uses for what it is working on at the time, so if you want to work on a document it's loaded from the hard drive into RAM so you can work on it and then when you click save any changes you've made are written back to the hard drive. so the more RAM you have the more things you can have open at the same time without it effecting the performance of the computer. RAM is very fast.
In theory the larger of either is better.... but realistically speaking... a 750GB hard drive is pretty large and 6GB of RAM is plenty for most people... so either would probably do you.