First, take a look at my Mac Malware Guide. That will give you more information about what threats are out there and how to protect yourself.
10.5.8 is a bit old, and is lacking most of the protections available on newer versions of Mac OS X. So the need for anti-virus software may be a little greater than on more recent systems. You should be able to use either Sophos or ClamXav on that system, both of which are quite good. Even better, they're free!
I have Mac OS X 10.5.8, what virus protection is recommended for this system?
I would give first consideration to upgrading your OS as high as possible before spending a dime or much energy on additional malware protection. In addition to Thomas' great guide, you might find this new article of interest TidBITS: Do You Need Mac Antivirus Software in 2013?