Soak noodles in lukewarm water 10 minutes, drain; peel apart into individual strands. Poach shrimp in chicken broth until just cooked; peel, devein and halve lengthwise.
Blanch whole bunches of choy sum in boiling water; transfer to a bowl of ice water for 30 seconds, squeeze dry, and cut into 2 to 3-inch pieces (5 to 7 cm.).