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.