1. HDD with bigger capacity = ability to store more stuff on it. Yes, generally cost is relative to capacity, but don't forget you may have to factor in shipping costs if you buy from online places such as Amazon.
2. Just download reaplayer from the official site, unless you have a need for an older version:
http://www.real.com/realplayer3. What is the make and model of the computer? think you were going to find out a little later, but just don't forget though.
4. Finally, you can just save your old hard drive, put the operating system onto there, and get another drive to store your data if you wish to do that. May be a good idea, see what other members opinions are for you though. Good luck.