Since I don't use PS8, I do not know the nomenclature for that product. I use PS Elements 2.0, and I rarely bump against any limitations. In Elements, using the drop down menus I selected Enhance\Adjust Color\Remove Color and that got it to a Black and White.
Then I selected Enhance\Brightness-Contrast and I increased the contrast to about +10 and lowered the brightness by the same amount. Some of the branches got washed out, so I then used the Lasso tool, and selected an area of the sky where the small branched were. I went around the large ones, as they were dark enough already. Once I had my selection, I went back to Enhance\Brightness-Contrast and further lowered the brightness and brought up the contrast so the small branches would be seen.
The one thing about PhotoShop is that the help system is quite good. If you just F1 and then choose Index and then Color, there may be an entry on Color Removal.
Rad