Is it worthy to turn on File Vault? I'm the only user of my MacBook Pro 5,3 (mid 2009, Core2Duo 2.66 GHz, 4 GB RAM).  

Is it worthy to turn on File Vault 2?  Any performance penalties considering my hardware specification? 

I'm running Lion (upgraded from Snow Leopard) and considering upgrading to Mountain Lion. 

What about my Time Machine backups? Should I preconfigure any thing?
Do you run the risk of loosing your computer? Is the information stored on your computer of high value to other persons?
If your answer is no to these questions you should not consider using FileVault 2 as it has some drawbacks:

The encryption happens on the fly. As your C2D processor does not support hardware accelerated encryption, your CPU load will increase. Newer CPU's like the Sandy-Bridge processors support that Apple ships support AES-NI instructions for hardware accelerated encryption.
Your I/O tasks will become slower for the same reason. Even witth AES-NI support the performance slightly decreases. But in your case the effect should be a lot bigger.
Auto-login is not possible with Filevault 2 enabled.

Time Machine
Time Machine does not encrypt backups per default. Even with FileVault 2 enabled. You will need to enable encryption with Time Machine. In Lion, FileVault 2 did not support encrypted wireless backups but work-arounds were possible. Mountain Lion supports wireless encrypted backups to Time Capsule.