You should be fine, Here is a link that shows 2 6970s + a I7 920 @ 3.3
It will be working fairly hard if you are going to push furmarks on it. The power supply may(will) get loud when you game(but the cpu heatsink will be WAY louder then anything).http://www.anandtech.com/show/4061/amds-radeon-hd-6970-radeon-hd-6950/24
You can always get a power meter from a local hardware store and check your current system power if you want an idea of what you are using now. That said, AC power numbers are always higher then the actual DC the power supply is pushing.
Numbers for a 6970. Just in case you unlock and overclock enough.http://www.techpowerup.com/reviews/HIS/Radeon_HD_6970/27.html