Find the one with the best specs & bundle within your pricerange... Yes you can get a bad card, but that can happen with every manufacturer, not just PowerColor or Sapphire...
And if you do get a bad card, RMA it and get a new one...
One thing you might want to be on the lookout for is what RAM is being used on the card you're thinking of buying... I seem to recall that PowerColor is cutting cost by using a bit slow ram, ATI a bit faster ram, whilst the others are quite good... But it's worth checking out what chips/speeds they are running...
(The better the chip, the larger the oc you'll be able to get out of it...)