I'd go with a 7870, and if you need more later you can buy another 7870 then. Better to go this way then to want to upgrade the 7850s down the road and have to get a totally new card. I've done sli on lower end cards and as awesome as the performance was I wished I'd gotten one slightly faster card I could double up on later.
If you do decide to use a pair of 7850s they don't use 600w each, thats the total system recommendation when using one of those cards, plus the rest of the PC. A good 650w or higher PSU is enough. And the standard cooling on the cards is enough to keep em cool as long as your case has decent airflow (a few intake and a couple out blowing fans).