I use blender 2.49, so I briefly looked at 2.6... which I think is still similar to 2.5.
1. In Edit mode, make sure the sculptie's middle vertexes are at x=0
2. Delete all the other vertices on the side you don't want to mirror.
3. Select the vertices at x=0
4. Hit shift + s and select "Cursor to Selected"
5. In Object mode, go to Object -> Transform-> Origin to 3D Cursor
6. In the buttons below, go to the wrench button. Click Add Modifier -> Mirror
7. Select-
Axis: X
Options: Merge, Clipping
Textures: U