minimum catch all would be 550w if you want to add more gfx cards later then add 200w per card (amd)... better to buy a bigger psu now, than having to replace a perfectly good low wattage unit when you want to upgrade.
dont bother with anything over 850w as you wont need it unless your throwing 3-4gfx cards in.
Well im getting a 750 - 850 watts but which one would be better cause i was reading that Corsair is very sexy on PSU's but i can get for almost the same price a Cooler Master with 100+ watts so this are my options atm:
1 Cooler Master Silent Pro M 850 w $162
2 Corsair TX 750 w $154
3 Cooler Master GX 750 w $121
4 Cooler Master Silent Pro Gold 800 w $176
5 Thermaltake TR2 RX Modular 850 w $176