if you change your cpu yes you can sli even 780 but amd processor is power ungry. (intel 1366 and 2011 too)
270x 6+6 pin+pci = 225w each 450w for crossfire more 100w (i5 i7) 1155 1150 = 550w
Thank you, i do understand that my CPU is power hungry! 125watt TDP
After hours of googling i found out that the R9 270X need about 300watt in CF full load ( and that is with all components ).
125+300=425 so i have 125w headroom?
Can you confirm this?
EDIT: after investigating your answer, you say with Intel CPU i can SLI 780?!
780 cards have a TDP of 250watts? 500W already.
So the lowest PSU you would recommend is a 650W of a good brand? or 700+?
And whats the difference between a good 550W (my XFX PRO 550) and a budget 650W (for example a Corsair VS650) ?
The SLI configuration of 270x changes everything due to the components already in your system as it will increase the load on the PSU past 80% of its capacity which is not good for your components nor the PSU.
The XFX is capable of running 2 radeon cards however that would be like running the 290x which is already 2 radeon cards combined on one PCB combined with the rest of your components would run at over 80% of the XFX 550's capacity whereas optimal capacity is 50% of PSU's capacity or 225 but it can go up to 400W when under full load however that should not be the continuous power draw.
It would probably be easier to just ask for recommendations for the best 750 to 850 W power supply to run 2 r9 270x in another thread since most of PSU experts won't bother commenting on a thread that has 5 or more comments that they hadn't replied to previously. (They don't want to read all of the other responses to give a better answer fwiw)
Also , I'm not looking for a PSU upgrade just now. I was wondering if its possible to do 2xCF without replacing it.
Since it seems barely possible/ pushing the limits of this PSU, maybe its better to sell my 270x and get a better single card solution without changing PSU?
If you don't mind the hassle of selling the r9 270x then getting a better card is a good choice if you have the money and need more processing power however wait until you get the better card and double check everything before selling the r9 270x.
Quick Summary: Add up all the max TDP of every components and it should be between 40 to 60% of a quality PSU's maximum wattage/ capacity. PSU selection should be calculated by maximum load times 2. 400W Max TDP would require a quality 800W PSU.
For a system using two AMD Reference Design Radeon R9 270X graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 700 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 43 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) may require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
The +12 Volt continuous current rating of 43 Amps or greater, that I specified above, will handle the running of FurMark stability testing and GPGPU usage with two overclocked AMD Reference Design Radeon R9 270X cards. It should also be sufficient to handle the overclocking of the i5-3570K to 4.4 GHz.