Not that you asked me, but all of my video cards are 5970s on Linux (LinuxCoin+SmartCoin).
You can definitely control the voltage on 5970s with AMDOverdriveCtrl. I use it to undervolt. However, I do not know about overvolting. My MH/s per watt ratio starts dropping above 875MHz using stock volts. Overvolting would only decrease that ratio for me and add excessive heat. You might have different results though.