Yeah It does get confusing sometimes with all the options,but the dallas 420 has 16K of flash memory which is just code memory,because of the way the 420,and any other 8051 derivative has been designed you cannot mix code and data spaces as you can with other designs,the movx instruction will not work on the code space.
The 420 has 1K of internal sram which you can use for data,and the movx instruction will work on.
Programmable through the serial port means that you can connect your pc's
serial port to the 420 serial port and the 420 will use an internal routine to program the flash with data it receives.

And datasheets are a neccesary evil I am afraid.You should see the CD roms full of the things that I keep getting sent.
Hope that helps.