Which drivers are you using? On Catalyst 13.2 and 13.3 beta, I can pull 550MHash/s+ per card under BFGMiner. Also, make sure your power supply is sufficient and in good working order, and install MSI Afterburner to check temperatures and that the 7970 is being utilised properly. (When mining, GPU usage should be pegged at ~99%. Significantly lower values suggest that the GPU is being underused for some reason, and square waves suggest VRM throttling.)