Call ATI tech support, confess what you did and see if they will attempt a repair for a flat fee, like $35-50. MSI has a deal like that on their stuff. I said attempt because if you ripped up any of the circuit traces or pads when you pulled off the part the board may not be repairable. If you do any additional damage trying conductive epoxy or soldering yourself they won't work on the board.
Now the technical stuff:
Don't even try to fix it yourself. Initial soldering on surface mount devices is done by screening a solder paste onto the board. Placing the parts on top of the paste and then running the board through an oven that flows the paste. Its a "low" temperature process that makes for weak mechanical connections (but you know that now).
Repair techniques for SMD involve heating the entire board to a high (but safe for the parts) temperature and then using a hot air jet to locally heat the solder past above it's melting point. The work is done under a microscopic viewer so you can actually see what you are doing with the tiny packages.
A conductive epoxy to reattach the part might work, BUT!
The epoxy will be a higher impedance and electrically noisy compared to the solder pastes so if the part is a high speed device it may not work or not work reliably.
The risk is also very high that you will slop some of the expoxy onto another conductor shorting something out. If it's a power lead thats shorted the card will be toast.
I used to do this kind of work years ago and wouldn't even think of trying it without the proper tools (that's about $10k worth).
We don't even have that kind of equipment at work (15,000 man aerospace plant) anymore. We farm the work out to specialty shops. Shame we don't have the equipment, could have used it to unlock my Athlon XP.