If you don't want to do a new clean install of W7, which is the easiest option (provided you don't mind reinstalling your apps), then you can use any of the partition management suites to copy the W7 partition across to your new HDD. If you don't already own something like Paragon or Acronis, and don't fancy buying software, you can look out for free copies bundled with PC mags (there's one virtually every month in the UK), or search the web for "freeware partition manager". There are several out there.
Once you have cloned the W7 partition, you'll need to disconnect the old HDD temporarily, boot the W7 DVD and do "repair your computer" "startup repair" three times. That will completely recreate the boot files on the new HDD.
Then keep the new HDD as the first HDD in the BIOS boot sequence when you reconnect the old HDD (if you intend to) and reformat it.