The damage dealt by the target Phoenix as Syrix's first triggered ability resolves is equal to its own power, not the power of the creature card that left the graveyard this turn.

You must pay all costs for Syrix when you cast it using its last ability and you must cast it as the ability resolves. You may not wait and cast it later in the turn (unless of course the ability triggers and resolves again).

If Syrix dies at the same time as another Phoenix, its last ability won't trigger. It must be in the graveyard already at the time that the other Phoenix dies.