The XFX 5870 doesnt have voltage control due to XFX being cheap and removing components form the VRM.
EDIT: If you are planning to buy hardware now for mining you most likely didnt understand how bitcoins work.
So now what's the correct plan in line with how bitcoins work? I'm new here and don't know it either.

The return on mining is going down 40% in a few days, and will continue to go down. If you are buying that card solely for mining it will take longer to pay off. It will still probably pay off but it could take a while.