GeForce GTX 570 in 2-way SLI
A second card requires you to add another ~225 Watts. You need a 750+ Watt power supply unit if you use it in a high-end system (800+ to a KiloWatt is recommended if you plan on any overclocking).
If you look at my public profile, you will see that I run x2 570 SC's in sli with a 3.75ghz clock on a i7-950 myself. I run 811W-830W running full load. Meaning 3dmark11.
I have a UPS and a wall wattage meter that tells me exactly how much i'm using. With only my computer, (not my tv/monitor or any peripherals), I get the 811w-830w reading running 3dmark11. I run a 1050W corsair HX myself.
So in my opinion, 750w is not nearly enough. The 2 things that most people seem to forget is the efficiency of the PSU and capacitor use/wattage requirements over time.
So I run a 1050W PSU. My actual sustainable power output is 924w. This is because my PSU is 88% efficient. Now some PSU manufactures under-rate their PSU like PC&Powercooling do. (Their 750w is really 825w.) But One doesn't know that information without many hours of research.
Assuming that PSU makers post the real numbers, like say 1050w, then I know my real max power output is 924w at 88% efficiency.
The other thing I mentioned was capacitor needs. over time, most computer parts need around 25% more power over about a 5-6 year timespan due to the in-efficiency of capacitors charging and discharging so many times. This applies to all components. I'll explain why this is important in a second.
Computers can run in many power states, but the following are the most common examples:
Here's where things get interesting. I use the following rule with PSU's.
RULE: Gaming need + 25% + 25%.
Here's why. If I need 565w maximum in the heaviest games, I'll also need to account for 25% more wattage needed as the computer ages. Additionally, I don't want the PSU to run at it's peak all the time because that makes heat and strain on the PSU that can be avoided by just getting a bigger PSU. This is where the extra 25% comes in.
So my needs are 565w + 142 + 142 = 849w. We will call it 850w because it's close enough. The first 25% is for wear and tear of components just needing more power over time, and the second 25% is a cushion so your not taxing your PSU all the time. Things get fun however if your start benchmarking all the time. Then my needs go up to 830w at peak. Which also show's a need for 850w.
If you don't care for the cushion or the 25% extra component usage, then 750w will work. It's just not something I advise.
One thing you should do is to go get a wall wattage meter from fry's for $15 bucks and plug your computer or powerstrip into it and game/benchmark. See what you output really is at peak. (don't connect you monitor/printer/router etc etc to the meter/powerstrip) during testing. We only need to know the output of the box.
You will need less wattage if you get a more efficient PSU like the seasonic 1050w because it's 92.8% i believe vs mine at 88% silver vs gold certification. (not sure.)
I mostly sprang for mine because of the 7 year warranty and room to tri sli or load my computer up without needing to worry about power.
On your other question regarding the OCZ PSU, i'm not a big fan of OCZ anything, so i'm biased. (bad ram, not the greatest ssd's, etc etc.) So I have no opinion. But I would say that corsair/seasonic are very reliable ways to go.
Your very welcome for the response. I do enjoy detail!
Well, yes and no. The attached cables are what you would need anyway, a mobo 24-pin, a 4/8 pin (also for mobo) and 2 (6/8) pin connectors for a single videocard. You need to run those cables anyways, so in that regard no. But I did have to RMA my PSU once to corsair and it would have been easier to just unplug the PSU only leaving all the cables. So in the regard yes.
BTW, corsair is freaking awesome with RMA's. Case and point, I only buy corsair ram and I use the corsair liquid cpu cooler the H70 . Didn't care for there cases, just not enough cooling for my 570's.
But if I had to do it all over again, I would pick the seasonic 1050w. Full modular and gold rating for the same price as the corsair. I believe the seasonic is only a 5-year warranty vs corsairs 7-year. But that's a moot point anyways as computer parts are always faster/cooler/and use less power as tech get's better over the years. 20 years from now people will think 500w PSU are huge. lol.
If I can suggest one thing for your 570's. Go to EVGA's website and get the high-flow brackets. Dropped my videocards temps by 3-5c each! I noticed that when I ran 3dmark11, they would throttle a little due to heat and now they don't.
One thing I forgot to mention is overclocking headroom. You needs lots more wattage for your CPU if you decide to OC it past 3.8 in the future. I kept mine at 3.75 because I like my ram stable at 7-8-7-20 at 1600mhz vs 1650mhz and looser timings.
Just keep the OCing or tri-sli in the back of your mind when picking a good PSU with enough power.
One last thing. Keeping in mind my last post of my 830w peak usage when benchmarking, if your OCed your CPU more and OCed your 570's ever, 850w will not be enough. You will exceed my 830w by quite abit. Thus making a 1050w a requirement and not just a (I want because ....)
Take all my advise with a grain of salt my friend. I know much, but don't have all the answers!