g_dismember is server-side command, its supposed to be set to 100 I think, not sure on that 1. I do know it can't be on 0 for dismembering to work.
cg_dismember is the client-side
cg_dismember 1 turns dismemberments on, but its like a kiddie kind of dismemberments, it won't show someones heads being sliced off or something like that.
cg_dismember 2 is full dismemberments. See people get sliced & diced every which way.
In Single Player
helpusobi 1 should enable cheats, then
g_saberrealisticcombat 1 should enable dismemberments, I don't play single player much so I don't know much about it. I think there may be a 2 option with this just like multiplay, again, I'm not sure.