can I update my Mac osx 10.5.8

Not directly, no.

Before you buy anything, you must first make sure your Mac is compatible.

For the minimum specs for Snow Leopard read the technical specifications … Mac OS X v10.6 Snow Leopard

For the minimum specs for Mountain Lionread the technical specifications. ... Apple - OS X Mountain Lion

All being well, buy your disc.

Once you have bought your copy you will need to update via software update or the Combo updater for Snow Leopard … Mac OS X 10.6.8 Update Combo v1.1 Remember to repair permissions both before and after each upgrade/update.

Once Snow Leopard is installed and updated to 10.6.8 you can then progress to Mountain Lion.

You can no longer buy Snow Leopard from the Apple retail stores. If you are in North America ... the last I read, you must call the on-line Apple store at (1-800-MY-APPLE). It is $29 plus tax for family pack and $19 plus tax for a single. Otherwise you can call your National Apple Helpline and ask for Sales. http://support.apple.com/kb/HE57

actually my question is--should I update my Mac os x 10.5.8 to latest version, will it be compatible with macbook i have, will it be more beneficial

Please start your own thread. This one is outdated and the OP never followed it up anyway. Ask the question in the Leopard columns and be sure to give details of both machines and their current ISs.