My friend wants to crossfire 6870s but the thing is that I'm worried the CPU (AMD FX 8120 @ 3.5.GHz) will blow his 600w PSU (It's Corsair, Don't Worry).
Will it blow?
look online for the max wattage for both cards and the cpu. then devide the wattage by 12 and that the amps the parts will draw. take that number and look at the 12v plus line make sure it lower the nthe 12v line or you burn out the power supply.
CX600 shows 46A on the 12v rail, and your looking at about 40-46A total, that's cutting it real close. Here's the (rounded) math, each HD 6870 has 151W TDP (~13A each) the CPU has a 125W TDP (~11A), the fans, HDD(s), optical drive etc would probably come close to taking up the rest - I know I wouldn't try it
Yeah, it will probably be too much or even if it works it will be almost fully loaded. It's not healthy for the PSU and not worth frying your components when it fails. Quality 700W with adequate amperage on 12V rail is required.