Wow this thread's going donwhill fast. Like they said, get HL model viewer and also you're going to need a program called wally. I think you can use wally to pull the textures out of .wad files, then import them to PSP, edit them in a way that's suitable for you, then repack them into the .wad file in wally. You can use HL Model viewer to see how it looks in 3d.
-Karrde-