I have 2 flash games (written in as3). Both the highscore value being hacked. The normal range of each game score is not more than 5000 (normal users, will only get 2000 - 3000
You could to add some sort of RSA (asymmetric) encryption to your Flash apps. You would put the public key in your Flash app, and use the private key on the server side to decrypt the score sent.
This won't necessarily prevent a hacker from hacking your Flash App, but it will make it a bit more of a pain in the butt.
Here is an AS3 library for RSA encryption.
Outside that it'll depend on what server technology you're using (PHP, .Net, or whatever) as to how you implement the RSA on the other side.
It should be noted too that this question has already been answered.