Ok heres the run down
The 480 is a single freq 1500 watt machine
The 580 is dual freq 2000 watt machine
both are gps and both are grayscale pics ---nothing wrong with that as I have found some times the black and white picture outperforms the color units.
also included in this price catagory would be the Garmin 400 dual freq unit with greater power @4000 watt
So whats my choice
none of the above hehehehe :

:
I really like the humminbird products and own two, one of the main reasons is it is the only fishfinder that will give you both the little fishy symbols and arches on the same screen
My selection for you in that price range is a humminbird 737
http://www.humminbird.com/products.asp?ID=653it looks a little higher at cabelas and bps price wise but it just over 300 on ebay---jump quick -- one left
bad news you'll have to get you a gps antennae for it
good news I got an extra on the easy payment plan
this is fully expandable to accept the navionics chips and adding your own card to be nearly as good as you will ever need for cat'n
it also has the plus of 90 degree viewing which works really well on bait fish and crappie
in reality its an upgraded version of my Matrix 37 which I love with the expanded ability for the gps mapping chips :

: and screen snapshots.