The 670 adds on to the 660 with the addition of build-in European as well as North American mapping data -- something that will come in very useful on my upcoming Liberty Alliance Technology EG meeting outside of London (in Ipswich). Last time I drove in the UK it cost me £10/day (about $20/day) for the GPS system from Hertz -- and it really helps with all those roundabouts.
Otherwise, the 670 is pretty much the same packaging as the 660 -- in fact the only way to tell the two apart is to pop up the GPS antenna and look at the serial number label underneath. They do add European adapters for the power plug, but since I just charge off my computer and/or the car, this doesn't mean much for me.
There is a non-trivial difference in the price, but if you do some traveling to Europe, having the map set built in is well worth it.
If you're interested, I'm selling the 660 on eBay -- it's a good unit if you primarily drive in North America.