Ivy Bridge i5s are 77W I believe, but let's round up to 100W. 7970s are in the range of 250W I believe, but let's round up to 300W to be safe. So 400W for those two parts, plus ~50W for the motherboard and ~50W for the rest of the system is 500W at 100% usage of all parts (never happens). A 500W PSU with 33A on the 12V rail(s) should be enough for your system under worst-case.
Even being super conservative, your PSU -- with 45A on the 12V rail -- is plenty.
you really don't need 1000w, a 600w from a good brand will be enough and by good brand i mean Antec, Corsair, Coolermaster, Enermax or any one that use Seasonic or FSP internals)
I personnaly wouldn't buy an OCZ unit...
PS: I use about 450w at plug on my old rig (measured with a kill-a-watt) with a Q6600 at 1.6V vcore and dual 560ti DCII TOP and 6 HDDs so you don't need more than that for sure as newer tech consume less power... My new rig (see my signature) consume about 250watt in 3dmark and with Furmark and Prime95 to max out all at the same time it's about 290-300watt without my raid array and 380-400w with it... PSU tends to be more efficient at 50% load so that's why i suggest a 600w for you...
I take a 1000w as i want to do SLI and i have a bunch (12) 1TB HDDs (it was the top capacity when i buyed them) for my Videos on a raid 10 array...
At plug is greater than the PSU usage as it has a efficiency difference ex: a gold is 90% so deduce 10% from the amount of current drawing from the plug to have the real power consumption of your rig...
For a system using a single Radeon HD 7970 GHz Edition graphics card AMD specifies a minimum of a 500 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 42 Amps or greater and have at least one 6-pin and one 8-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) will 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 OCZ ZT Series 550W (OCZ-ZT550W), with its maximum combined +12 Volt continuous current rating of 45 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with a single Radeon HD 7970 GHz Edition graphics card. This model doesn't give you much, if any, spare capacity to handle overclocking of the CPU and GPU and survive running FurMark.
Also the Max TDP of the 7970 ghz edition (the fastest one) is 250w + 77w for your 3570k + 10w for the ram + 10w for an HDD or 2 for a SSD = maybe about 350watts and all that it's the maximum possible if all these components are Maxed out so you don't need more than a 600w as at 50% to have the better efficiency that only give a 6-700watts MAX...
If you plan to overclock or major HDD array or further expansion like SLI/Crossfire, then pick a higher model 250watt higher for another 7970 ghz edition and about 10w higher per HDD/or other device... The overclock will bring the I5 to about 100w @ 1.35V (3770k at 1.35V for reference is 28watt more at the plug for me) and GPU don't scale like that except if you highly rise the Vcore (i only see about 30watts difference on my dual 560ti from 925mhz to 995 for one and 925 to 1020 for the other card)
So in clear I5-3570k @ 4.8 1.35V (if you have a good chip as it's more a 1.4-1.45V case in some chips) and a 7970ghz edition would consume about 350watt and you add your HDD ram and other peripherals (all usb device used) and you'll get about 400w like me with the overclock so...
So until I decide to OC my PSU should be enough right? When I do decide to OC what size PSU should I be looking for?
Yes, it is enough. realhardtech suggests 550w and that has some headroom built in.
You are OK to go.
I do recommend upgrading if you are going to OC regularly. You don't need 750w
unless you are going to CF 2 - 7970's then you need 800w. I would say
a quality 620- 650w unit would give you plenty of headroom for OC.