[ QUOTE ] use a 1024 x 1536 map (unusual size, yes, but still a power of 2) [/ QUOTE ] no, 1024x2048 would be power of 2. Always "double", you cant just sum with another power of 2 number.
but only newest hardware generations support non-power-of-two textures, and they are likely being a bit slower, and also often are not fully supported (come with useage limits on filtering and so on). And 1536 remains to be a non-power-of-two :P and Eric's linked article is a classic, it should be sticky must-read for…
Why would you need different UV channels for those pieces? You only need different UV channels when the same geometry needs different uv-mapping (like Crazybutcher suggested with lightmaps for example) - unless you mean different materials for each part. All the different maps could all be on the same UV channel though.…