There's not much other than the increased power the SB800 has, the fact that you can use additional power packs for quicker recycling, it comes with colour correct filters, bounce card and it can act as a wireless flash controller on cameras without that ability (though you D80 has that).
Plus there is a third party addition to the SB-800 that turns it in to a ring flash.
I personally bought a SB-600 when i bought my D80 (11 months ago) and where as it's done everything I've asked of it there are times when I've regretted not spending the little extra on the SB-800, just for the extra power, or the colour correct filters.
Whatever you decide Kerso on here is the place to get either, I believe the SB-600 is £139 delivered and the SB-800 is £199 delivered bu you are best asking him direct.