invisik :
a single 4870x2 i would recommend a good quality 600w and 750-800w for cf. 1000w overkill.
if your building a new system as i am with 2 x 500 gb hdd a quad core cpu 4 gb ram and a 4870x2 i would go for the 1000w psu. only because if you decide to stick another 4870x2 and more ram plus another hdd or 2 along with sound card, tv capture card or any other pci component not mentioning usb devices you would stll have more than enuf power in reserves,, meaning your psu is not working as hard which means it will last longer with a nice balance of power all round your system. If you can afford a 4870x2 then the price of a 1kw psu isn't to bad to make sure you have a steady flow of power going around every component in your system.
well thats my advice and thats what i'm doing even though the 1000W psu was about £40 more than a 850W.
This is my build to date just waiting on mother board and gpu to build should be here in around 2 - 3 days hopefully for the weekend.....
INTEL Q6600 SLACR 2.40GHZ
MAINBOARD ASUS RAMPAGE FORMULA X48
SOUND INTEGRATED
VIDEO RADEON SAPPHIRE HD4870 X2
RAM CORSAIR TWINX XMS-2-8500 2x2GB
HDD 2 X 500GB S-ATA 16MB
THERMALTAKE TSUNAMI VA3000
PSU THERMALTAKE TOUGHPOWER 1000W
thats without my usb devices and heatsink fan plus 2 extra 120mm fans for extra airflow plus more smaller fans dotted around i may even decide to go 8gb ram and stick another 500gb hdd in yet i don't know but a 1000W psu will allow you to add more devices without a fear of you psu not being able to handle the power consumption and still having a nice regular even flow across components and not having to find out that because you added a usb device or pci device extra you have to rush out and buy another psu to cope with the extra power needed.