Asus and Gigabyte are the most expensive of the cards listed on their website, and aren't worth the premium IMO.
The Leadtek's better value than either the Asus or Gigabyte, but i'd just save the cash and stick with the Forsa.
If you look a little further down the video card section there's actually an Albatron Exteme 7900GT listed seperately (below the GTXs) that's $30 cheaper again. That would be my pick.
For anyone who's interested, current prices from MSY (all in AUD) are:
Albatron Extreme: $389
Forsa: $419
Leadtek: $435
Gigabyte: $475
Asus: $475