Wow. Literally every suggestion in here has been wrong. 900watts for 3x 5850s? lolz. Only run your PSU at 50%?
Nobody in this thread suggested "only run your PSU at 50%".
Here's a real rule of thumb: Get a quality PSU and run it up to 100% of its rated continuous power output.
You're going to take responsibility if somebody's quality PSU blows in a few weeks and maybe take out a component or two? Giving the max isn't the same as giving responsible advice. We told the OP the various factors, outline the min/max scenarios so that he can make his decision how fine he wants to cut it or how safe he'd prefer to play it. Giving an unqualified "get a quality psu and run it up to 100% rated" isn't responsible. You're overlooking factors like thermal derating and manufacturing variance.
I heard a really good anecdote once -- Buying a cheap PSU is like buying a cheap condom. Seems fine until you have a blow out.
For a mining rig, a quality gold rated PSU will literally pay for itself over say, a decent bronze rated psu, if running 24/7.
I've become a bit of a fanboy of the seasonic X series, though they're not as cheap as they once were. You will likely be fine with an x650, though if you're super super paranoid you can go with say an AX750, such quality it is (and basically the same unit as the X750 with some minor alterations). Also sweet but too expensive is the NZXT Hale90, but looks awesome. Lower quality but still respectable units are the OCZ ZX series, Thermaltake ToughPower, FSP Aurum series, Enermax Pro87+ series, Lepa GXXX series. Not as good units but if you can find a bargain they'll generally be good enough. I'm currently running 2x5870s and 1x5850 on my seasonic X650 right now, as I recall it pulls something like 580Watts from the wall, full load overclocked. That's roughly 500Watts to the system, don't listen to anyone that tells you a 5850 is going to pull 200Watts from the PSU.
What you pull is only applicable to your rig. We don't know how exactly he's going to run it, how overclocked, how much overvolting. Again, it's a question of irresponsible and responsible advice.
Using your condom example it's like saying "I can reuse the same condom a few times and never caught anything so just do that". Sure the same condom would probably last a couple of fucks, but who's going to be responsible if somebody follows that advice, get a break and get a STD?
A quality powersupply labelled "XXX Watts" can deliver XXX Watts to the system. Which means it can pull XXX + XXX*efficiency watts from the wall, and deliver it all day long 24/7. Now if you cheap out and get some no name cheapo-brand 10000 watt PSU, you might get enough watts, you might not, it might blow and with no OCP destroy your whole rig, or it might not, no one knows. I prefer to play it safe myself.
It's XXX / efficiency watts from the wall, not XXX + XXX * efficiency.
OCP doesn't stop a PSU failure from taking out components, it depends on the actual failure mode. Having deployed literally thousands of systems with quality PSU from various manufacturer including Seasonic in one of my previous jobs, I can tell you it's 100% that a good PSU with all the protection mechanism can avoid collateral damage. Just like not every cheapo PSU will cause component damage if it pops.
I'm not saying he can't try to run 3x5850 on a 650W, it's definitely theoretically possible. But just don't make it sound like it's a guaranteed sure no problem configuration.