The difference between grace and karma, as i understand it:
karma is a principle ~ "what goes around, comes around." but karma won't give you favor with God. we can be nice people but being nice or good isn't the same thing as being righteous or holy.
that's where God's grace, through jesus, comes into play. we all need grace as a starting point in our relationship with God and with others. we need to be gracious in our relationships with our fellow human beings. when we experience true grace, then we can be gracious. our graciousness looks like karma. as we show grace (acceptance, love, forgiveness, etc) towards others, we'll receive the same in return. or as Jesus said, "Do unto others as you'd want them to do unto you."
God gives grace freely, but we must willingly receive it. it's a gift, never earned. it's received in repentance (or turning away from selfishness and towards God). if we don't receive God's grace, i believe, then we cannot properly share true grace with others.
don't know if that makes sense, but i think you know what i'm getting at, right? i welcome your thoughts.