SP is a problem yes, but for MP this is ALL you need:
g_dismember 100 (this is probability that it will happen, 100 is 100%, ie: all the time with a saber kill)
cg_dismember 1-3 (3 is full dismemberment, though 2 might even be though for that, 1 is minimal, 0 is off)
My problem before (haven't tried it since reformatting) is that on the last two levels on the "Dark Side Path" dismemberment stops working on enemies and only works on ME (the player). That's with the 1.01 patch.