What power supply do i need for 2 HD5850 crossfired

Well, what is your budget?
I would suggest you this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Yes, that's 750W, more than enough for dual HD5850, but with that PSU i'm sure you will stand for a long time before buying a new one...  

"Coolermaster" are not a good brand (although they are not the worst brand). id say a 700-750w good brand PSU, corsair, antec, silverstone, that will give it plenty of breathing room. The antec truepower 850 is more than enough. I think there is a "truepower NEW", which is the one to get, not the older model.

While that is a great PSU a 650w by Corsair should be more than enough. This one is a very nice deal;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Just $51 after the promo code and rebate. Can't beat that. If the OP really wants a 750w of a good brand despite that is really isn't necessary then this Antec unit is a good deal;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

^+1
Great deal, jyjjy...  

If your from the UK overclockers UK have got the corsair tx650 for 75 quid, just got it myslef, runs my 5970 perfec so till will easily run ur 5870 crossfired. as dip said above OC it youl need 700-750W.

http://www.amd.com/us/products/desktop/graphics/ati-rad...



If overclocking, I'd opt for a Corsair TX-650 and either an Antec EA-650 or TP-650. If doing serious overclocks, I'd opt for the Antec Signature Seasonic X-650 or Corsair HX-650 as a minimum. Check here to see just how much of an Overclosk ya can get away with:

http://www.antec.outervision.com/

For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 02/26/2010 and ones w/ double asteris (**) made it onto silentpcreview.com's Editor's Choice List:

Note: Just because you see a brand name on this list, never make the assumption that their other lines are of matching quality. Silverstone for example has other models w/ 6.0 and 7.0 ratings.

Antec Signature 650W (10.0) OOS **
Seasonic X-650 650W (10.0) $160 **
Silverstone Olympia OP650 (10.0) NFS

Thermaltake Toughpower QFan 650W (9.5) NLA

Antec Earthwatts EA650 650W (9.0) $75

Antec True Power Trio 650W (7.5) NLA

XFX XPS 650W XXX Edition Power Supply (7.0) NFS

Antec NeoPower Blue 650W (6.5) NLA

BFG BFGR650PSU 650W (6.0) NLA

NorthQ Black Magic Flex 650W PSU (5.0) NFS

NFS - Not For Sale / NLA = No Longer Available / OOS - Out of Stock