WWW or without WWW

My site pr=2 has all pages indexed without the www version .
It is also ranking good for many urls.
Should I redirect naked domain to www one or should i redirect www version to naked one?
Or leave it as it is?
For which one should i build backlinks?

You should always use www.

Without the www it's not clear from the address what type of resource users can expect to find at that address.

If you receive a request with the www missing, use server scripting to add it and then 301.

You can also set Google to always use the www for a domain through their Webmaster Tools service.