1. a good 400w power supply should already have 30 amps on the 12v rail
2. the ATI specs state 30A for single card, and 38A for crossfire.
3. yes, generally the 12v rail is the only one you need to worry about (the other rails will pretty much have more than enough power if the 12v rail is high)
as far as good brands go. I personally would recommend Antec. other reputible brands include Enermax, fortron, PCP&C, and seasonic. The reason I like antec so much (aside from the high quality and reliable protection circuits) is because they rate their PSUs actual power output accurately.
if you are willing to spend $100 and want an awesome PSU, then check out
this beast!, it's got three (yes 3!) 12V rails at 17A each for a total of 51A on the 12V rails. and with modular cables and a single 80mm fan @ only 18dB... what doesn't this PSU have or do??? Antec rates this PSU at 500w, other PSUs with as much power as the NeoHE 500 would be rated much higher (i.e. 600w or more)
hope this helps