Do n't bet on older tech not drawing much power, the Prescott Pentiums were notorious power hogs and some of the earlier X2 Athlons could chew through 120W with ease.
Mousemonkey is right about the 12v rails, according to the specs you have given the maximum combined output of 385W equates to 32 amps (amps=Watts/volts and 385/12 is 30.083... amps).
According to this site the card draws about 90W:
http://forums.atomicmpc.com.au/index.php?showtopic=264
Even assuming a heavy CPU draw you'll be just fine.
As long as the label is telling the truth.