For visualizing black and white conversion,
I like Russell Brown's tutorial
at http://www.russellbrown.com/tips_tech.html

It is very easy to adjust the hue slider to visualize
the effect that would be produced by placing different
filters over the camera lens

Russell also has a newer tutorial comparing B&W methods
of other Photoshop professionals.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=21027963