PP, the AGP 6800GS runs at the same speeds as the 6800GT. 350/1000. So unlocking the pipes makes it equivalent alone, no overclocking is needed. I believe the AGP GS is on the exact same core as the 6800GT, so the overclocking potential should be the same.
In fact, eVGA now has an AGP 6800GS that runs at 400/1100, faster than a stock 6800GT.