or do i have to use 24 bit ?
it depends on what you use as type. int requires normaly less memory than long, etc. (This can however be machine-dependend, but a long will never be bigger than an int, a float will never be bigger than a double)
In general: Yes 16Bit or 8Bit textures save memory. I would not use 8Bit textures anymore simply because of the colourtable hazle.
Of course lower precision means more banding.
BTW 24bit textures are uploaded into the videomemory as 32bit textures.
You may want to look into compressed textures.
compression sounds good.
how to use it ?
Or google for “opengl texture compression tutorial”. I’m sure you’ll get lots of hits.