You probably saw the binary trick for ROIs in 3dcalc's help. For the selective display, you can use the Instacalc function in the Overlay panel to restrict datasets to a particular value. The method would be basically the same as the 3dcalc command but without any permanent file on the disk. A similar function is available by right clicking over the Overlay colorbar and using the Overlay Tran 0D with the Expr 0D. Alternatively, you can use one of the paned color scales in the Overlay instead of the continuous color scale that is shown by default. Each pane can be selected to show a specified color or no color at all. Finally, setting the AFNI environment variable, AFNI_PBAR_THREE, to YES gives an overlay color bar that has upper and lower limits in addition to the threshold slider functions.
http://afni.nimh.nih.gov/afni/community/board/read.php?1,83335,83337#msg-83337
