My psu's real power output

In the past different voltages were more widely used. Now the two big power draw components (CPU and graphics) both use 12V. What this means is that modern, good quality PSUs are capable of delivering most of their "sticker" value in 12V.

So a decent PSU of 700W these days will likely have a 12V amperage of maybe 50A+, meaning it should be good for 600W+.
It varies from manufacturer to manufacturer, but you should really be getting 90%+ on the 12V rail.
XFX and Corsair 750W PSUs both have 62A 12V rails, so a theoretical 744W. That's the type of figure most PSUs should have these days, however a lot don't.

To get the sticker value, manufacturers add up the powers from each rail. On a cheap or legacy PSU, this means a huge amount of the power is basically unusable.
Yours should be 79W+150W+456W, for a sticker value of 700W. As you correctly concluded, it's the equivalent of a decent 500W PSU (roughly) though it's probably going to go wrong a lot quicker  

As for installing a GTX660, they really don't need much power. Nvidia says 24A but thats for the entire system, not the card. The card has a tdp of 140W, it's not likely to pull much over that, even if its got a factory OC.

So even though the psu is a piece of crap it can run a GTX660? Will it really produce 38amps from its 12v rail as stated or is that an exaggeration?

Please post the model of your PSU

It says in the op, arctic 700watt psu.
That's about it! Hehe!

That's the make, not the model. can you find a model number on it? Also..do you meant Antec?

For a system using a single GeForce GTX 660 (non-Ti) graphics card NVIDIA specifies a minimum of a 450 Watt or greater power supply that has a maximum combined +12 Volt continuous current rating of 24 Amps or greater and that has at least one 6-pin PCI Express supplementary power connector.

Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the critical factor.

The Alpine 700 Watt Quiet ATX Power Supply, with its maximum combined +12 Volt continuous current rating of 38 Amps and with one 6-pin PCI Express supplementary power connectors, is theoretically more than sufficient to power your system configuration with a single GeForce GTX 660 (non-Ti) graphics card. Whether or not this power supply is actually able to deliver the power stated on its label is another story.

Thanks, that's the reply I was looking for more or less, although it would be nice to know what its real 12v amp output really is, cos I bet its not 38amps at 90% load
I dunno the model number, its a cheapo make, and its not antec, I know antec. Its arctic

The Arctic and Alpine power supplies look exactly the same and I wouldn't doubt they are the same relabeled generic trash.

The only way of determining what it is really able to deliver is to hook it up to a PSU load tester. Doing that would probably result in the power supply's self-destruction when its limit is reached.

Safe to say it can probably manage 26amps on the 12v max then?

Chances are good that it will be able to.

The problem of those crappy psus, sometimes with 40-50% load, which will have with your system, that psu is a 450w-500w at the best, that's just fake labelled 700w, will have poor efficiency and high ripple and noise levels, that could lead to crashes, throttling, bsods.
That's why you should never cheap out on a psu, and not just look for the wattage of a one, but also the quality, efficiency,if it delivers what it's promised.

The reason you don't get 700W is that your looking at just the 12V rail. The MAX TOTAL output is 700W. The max of each rail will vary and they may add up to 700W or higher. Just make sure that the total load for each rail isn't exceeded, for the 12V rail you also have to include the power from any drives and your Mobo. The 12A difference you'll have could easily be used up by other components and may push your PSU over the brink.

Well I only have 1hdd, 1cd drive, Intel i5 3570, two led fans, sli capable motherboard by asrock and hopefully my hd7770+8gb ddr3.
That won't pop this psu will it?

It won't burn, or crap out.

Cheers, I'm gonna give it a go, if it explodes ill let you know hehe