@bryguy-
Don't use image->mode->grayscale
Use colours->desaturate
Then use colours->auto->stretch contrast (you might have to apply a small 1-2 px blur at that point if the histogram has too many gaps and looks like a comb) otherwise you will see banding when you bump map.
And re:grunge brush... I can't remember how it fit into this conversation :( Sorry :P
-Rob A>
