This has been the big question of security for many years. You are told repetitively that this is critical. Over the injunction not to trust anybody with your valuable passowrd(s) and not to write it down, it is interesting to know what would happen if a hacker tried to crack your password with minimal knowledge (the brute force attack as the experts know it).
One Man’s Blog has an interesting article explaining how easy it is to crack most passwords. The table above gives a rough idea of the frailty of most short passwords. 8 characters all in lowercase are found in around 2 days. I hope that your dat is not worth more than this small effort…
At the minimum, never reuse the same password on different web sites and for different usages.
Note: For once, I will advise you to use a Microsoft tool. Password Strength Tester freely evaluates the strength of your password.