According to AMD Support, 18 Amps minimum for an AMD system (whole system, not just processor). Most Dual-rail PSUs far exceed that going into 30+amps. Its really a non-issue, if you get a Dual-rail PSU, it'll be more than enough.
And the Antec Truepower 2.0 is one of the best PSUs out there, you could hook up 2 X1900XTXs to it and you'd be fine.
Just a FYI - a single 7900GTX system would take up around 400W at full load, so don't freak out.