Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I have bought a new iMac and it has a 320 GB HDD out of the box.
i need to upgrade it to 500 or may be 1 TB ine.
I mean physically
 
Upgrading the HD in an iMac is not for the faint-hearted, as you have to remove the screen. If you're not extremely comfortable with dismantling/repairing computers, I'd suggest taking it to an authorised service agent for the upgrade.

If, however, you're brave, you'll find iMac harddrive upgrade instructions here
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.