There are a number of ways. Have you heard of Carbon Copy Cloner
You can use this to make a clone of your current system onto your external HD. When you get your newly installed HD, just move the clone over.
(This glosses over some things, I believe, but you'll get the idea.)
I remember reading some time ago that Panther's Disk Utility allows you to do what CCC does so there's no need for it. But I'm not certain how this would work.
Good luck. Hopefully some of the smarter people here can help.