Blender Materials vs External UV textures???

Hi, I’m pretty new to the 3D world; I can model proficiently but i’m having trouble understanding texturing. Basically, i’m trying to make TF2 items. Now i can texture what i make in Gimp or something like that, and i can work with the materials panel to change transparency, speculars, mirror etc. But what i’m struggling with is understanding how they both affect the appearance of the model in different ways. What’s the best way to get a good idea of what the texture will look like on the model ingame? Should I enable GLSL textured solid, or just change the 3D viewport shading to Textured (Alt-z)? And regarding the material panel, will changing the specular intensity, diffuse and transparency affect the OBJ i export? Or does it exclusively affect the object’s appearance in Blender? And if so, what’s the best way to see what the texture will look like ingame? That’s really all i want to know; when do i use the Materials panel, when do i press alt-z etc.

While i’m here, I read someone stating that they applied an “AO pass”. Am I wrong in thinking that’s simply exporting a baked Ambient Occlusion and adding it as a translucent layer in an external software, or is it something else. Like I said, I’m pretty new to all this so bear with me.

I would be incredibly grateful if anyone could help me out, it would speed things up alot! Thanks, -U