RAM (Internal memory) is used by the computer to run programs. The more RAM you have the more programs you can run at once without seeing a slow down of performance. Also browsing the internet mainly uses RAM along with processor.
Hard drive size is the amount of data that can be stored on your computer / laptop. Every program you install takes up space on the hard drive so look for one with plenty of room.
Now, how much memory / hard drive space do you need?
It is 512mb of RAM (Internal Memory) that you should look for as a minimum.
You should be looking at a hard drive of about 60gb or 80gb although 40gb will probably be enough.
Stay away from Celeron processors. Go for Pentium or AMD.
Dell do some very nice laptops at quite reasonable prices. Have a look here:
http://www1.euro.dell.com/content/products/fea tures.aspx/notebooks_good?c=uk&cs=ukdhs1&l=en& s=dhs