Crossfire Power Supply requirement

620 watts for a Crossfire setup with two 4870's is cutting it very thin. I wouldn't suggest anything under 750 watts but then you didn't add in what other power requirements are as in how many drives and fans all that good stuff that would also effect power draw. A Corsair 750 watt unit should be fine. I'm using a Thermaltake 750 with my two 4870's and it works fine, if the Corsair would have been avalible when I purchased my PSU I would have bought one. As for Crossfiring a 4870 with a 4850 it will work but it would be better to keep with same cards either two 4870's or two 4850's.

All Corsair PSU's quality wise are excellent. But when it comes to running two 4870's the primary concern is output. Right now just looking at the web which the cards operate in 2D the current load from my system is 428 watts. Thats two 4870's, sound card (requires power from PSU), 4 hard drives, one optical drive, three 140mm fans and a waterpump. Now start up a game when it kicks in 3D and draws peak loads 620 watts isn't gonna cut it and my 4870's don't draw power for fans there watercooled. Like I said before you have to concider all the other power drawing sources. You can never have too much power but a lack will cause many headaches.

yes that is true, It's because the 520 & 620HX psu are made by seasonic i thought and the others are made by a different company

my usage on my PC would be:
1)overclocking q6600 to 3GHz
2)2x640Gb Seagate Sata II HDD
3)2xhd4870
4)xigmatek H-1283
5)Corsair 2x2Gb DDR2 800MHz 4-4-4-12

So I'm hoping 620Hx would be able to handle it well, would it not?

Well ATi doesn't recommend it either, The list the Corsair 650watt unit as only supporting one 4870, they suggest 750watt. I see they also updated the list to supported name brands and by wattage now.

I used to have a PC power & cooling psu but they are very loud compared to a corsair and a zalman