As in GPU-Programming, nvidia claims that GeForce 6 Series GPUs support 3D floating-point texture.
I got ‘GL_INVALID_OPERATION’ error with the following code:
The drivers may not support it yet. Also, a 16x16x16 texture at 16 bytes per pixel (assuming that it forces RGB to RGBA) costs 64MB. Perhaps the driver simply won’t allow you to absorb a quarter of the driver’s memory in a single texture?
<edit>
No way. I misread my calculator; it’s 64KB. Never mind.
As Nitro mentioned, use the ATI_texture_float formats, instead of the NV_float_buffer formats. The NV_float_buffer formats are only supported for rectangle textures.