Changing title of node when saving Whenever an user saves a node I want to prefix it with table autoincrement-id so that the id is unique.

So if the title of node is "xyz" then it should be saved as unique id "299999-xyz". Ideally this id should be the node-id which is unique but it may not be possible to know the actual id until the node is saved in the table. 

What is the best way to accomplish it?

I'm thinking of changing the title in the form validate function. This id can be numeric time() value. 

*PS:*I'll have tons of such nodes so when linking them from other nodes (node-reference) this id will uniquely identify such nodes in "autocomplete textbox" in node forms.
Use the module "Automatic Nodetitles". This allows you to use tokens on the title field. Once enabled you can set any pattern for the title. Set your pattern on /admin/content/node-type/YOURCONTENTTYPE to "[nid] - [title]" for example. Already existing nodes can be updated automatically.