I have OS X.4 on my MacBook and I need to upgrade being that my hard drive recently died and I must rebuild. Which operating system should I buy?

Please help me! I foolishly did not back up my hard drive which crashed and died last Thursday! I am now rebuilding everything including my iTunes Library which I have just discovered requires a higher OS to re-load previously purchased media. Can someone please tell me which is the next OS to buy? Forgive the ignorance - I am devastated and discombobulated! Word to the wise - back up your data!