The world base map that comes with the unit is very general - usually just used to zoom way out, pan to a distant area, then zoom back in to more detailed data.
Most of the mapsets here comsist of small quadrangles/segments of data in .img files.
You can send 1, or more quads, or an entire mapset(State) (or more) to the built-in memory or an SD memory card. All of NC would likely take around 170 to 200MB. 4Gb SD cards go for under $10 when on sale.
Take a look at some of the tutorial here on using BaseCamp and sending data to the GPSr.
If you are using Windows 7 on a 64-bit machine, let us know.