I intend to buy a HD4870 but I am not sure what is the power supply requirement if I am going to make it a dual configuration (i.e. 2 x HD4870). What is the minimum power supply required by 2 HD4870 under CrossFireX configuration?
and make sure your psu has enough pci-e 6 pin connectors too! , i have a corsair hx620 which is ok for one 4870 , but when i get a second ill have to get a new one (prolly 1000w corsair) as you'll need 4x6 pin connectors!!
Don't listen to them- they don't know what they are talking about. TH was able to SLI 8800GT's under a tiny (but good quality) 400-watt power supply. Look at some power consumption charts:
I would recommend at least 450-watts (though 400-watts should be enough, really, 450 would probably be better for CPU upgrades, overclocking, etc.) for a single HD4870, and 620-watts for CF HD4870's. People far overestimate how much power they will need in a system. Go to jonnyguru.com for more information, they are more suited for this type of question (the owner of the site is a technician for a PSU company....)
EDIT: I forgot to post that the 4 PCI connectors may/may not be a problem. You can use the molex adapters, that come with most cards, which I'm sure will work fine, but nothing is as safe as a dedicated PCI-E connector. Most 700+ watt units should give you 4 connectors.