1. My Voodoo 3000 card has 16 MB of memory. 256x256 RGBA texture takes memory (256*256*4=)0.262MB. Does this mean I can load (16/0.262=) 61 textures or is something else (like Windows) using the memory? 2. If I call glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 0, 0, 256, 256, 0) even once the frame rate drops from 100fps to 20fps! Is there a way to make the glCopyTexImage2D-funktion work faster?

There are other things using the memory, such as the frame buffer, depth buffer, etc, all depending on how you setup your pixel format. Also, the voodoo3 only works in 16bit colour, so it likely wouldnt store images as 32bit in video memory.