Part of the advantage of the Montana is memory. It'll handle any size SD card you care to put in it. The maps available from Garmin come on 4GB cards. The Montana will take a 32G so it should handle 8 consecutive 4GB Topo Maps.
And that's just the tip of the iceburg. I have a 660 and a Montana. There is no comparison.
"For those who fight for it life has a flavor the protected never know."
USMC 65-68 RVN 66-67