So if I have one texture map for a whole character, but I want some parts of it to have really sharp specular (metal/plastic), and some parts to have diffuse, soft specular (skin), can I just use multiple materials but one map? Or is the renderer going to load the map multiple times for each material?
Thanks in advance!

well thats what specular and glossiness maps are for - these control your specular behaviour.

so you dont have just one texture - in most parts you will end up with a diffuse texture (the colour map), a normalmap texture (geometry fake map), and specular&glossines textures.

just browse the polycount wiki to get some basic understandig how everything works.
http://wiki.polycount.com/CategoryTextureTypes

but its also possible to use multiple materials which is probably not ideal - but i guess it doesnt really matter in your case.

What engine?

A simple setup would be diffuse, normal, specular(brightness), gloss(size). That is 4 texture maps. Once each texture is loaded up they can be instanced around without using up more memory. With this you will be able to define your materials as you said.

[edit] Beat to the punch.

Ok, so it's this GLOSSINESS map that I need to check out.
I'm using Marmoset at the moment, and I made a spec map, but that didn't solve my highlight size issues.
Thanks so much guys!