CS is City Select, and CN is City Navigator. I believe Garmin in no longer going to sell newer versions of CS. and CN is is only option you will have.
See
http://www.garmin.com/cartography/ontheRoad/.
As for the MicroSD card, not Ultra-II card. I have the 76CSx and it came with a 128m card, and I have just purchased a 1GB card from Amazon for $42.
For your scenario, you may be able to put the first 500 and second 500 miles on the same card. It all depends on where the map is for. If the map covers a place where there are a lot of roads and POI, then it will be bigger. If the map covers a place where there is not too populated, then the map will be small. So, it all depends.
You can look at the maps online by going to
http://www.garmin.com/cgi-bin/mapgen/we ... 0&k=1&sc=1
You can look at the size of the maps online by going to
http://www.gpscentral.ca/accessories/ma ... av8usa.htm