>= 640x480 problems

I can't figure out how to use any video mode over 320x200 !  While in a
mode such as $5c, I write to memory at address $a000 but only the top 80
or so lines show up on the screen!

I have been searching for info as to what my problem is, and I think it
has to do with bank switching?

Anyone willing to help me with this one or direct me to the right place?

JW