I have VPS account with 256MB RAM.
But when Im trying registering, it takes very long time, and no email sent
Is this because I have too small resource?
256MB RAM is more than enough to run Drupal6. I run it in a 64MB VPS with MySQL as well. Have you optimised your service resources? You want to tweak lighttpd and fastcgi to be lightweight. There are lots of examples on the Internet.
It is more likely that there is something wrong with your mail back-end. Did you setup an MTA to send your mail? Alternatively, you may want to use something like sSMTP to proxy your mail over to a real mail server. PHP on its own will use sendmail by default. However, if your back-end MTA is not configured correctly, things can go missing.