Hardware NPOT Textures support is part of the OpenGL 2.0 specs.
According to Wikipedia,
"R300 and R400-based cards (Radeon 9500+ and X500+) are incapable of generic NPOT usage, despite allegedly supporting OpenGL 2.0 (which requires full support). These cards only allow you to use NPOTs if the texture has no mipmaps. NV30-based cards (GeForce FX of any kind) are incapable of NPOTs at all, despite allegedly OpenGL 2.0 (which again requires NPOTs). It will do software rendering if you try to use it."
Also,
Voodoo3 video cards (which were officially supported for WC3 by Blizzard) do not have an official OpenGL driver with hardware support for NPOT textures, according to the database of OpenGL Extensions Viewer 3.37