Replacing memory on a laptop is one of the easiest jobs you can do.
You can run a program here to see what exact memory you need.
http://www.crucial.com/
If your laptop has 2 slots then you would need to remove the existing 250Mb one and replace it with a 500Mb one.
You can leave the existing 500Mb one in there.
Note that sometimes using "mismatched" memory can cause problems, so if you want to be certain it will work (and it is not too expensive) you could buy 2 new 500Mb memory cards.
I have to say replacing your hard drive, AND copying all your existing data across is NOT an easy job if you have never done it before.
Whatever you do make sure all your important data IS BACKED UP TO CD OR DVD before you, or anyone else, tries to replace the hard disk.