750 watt overkill for this build?

750w on a 7850? a 450w could handle that
550w could handle a gtx 680

oh so I'd save money bringing it down some?

You could get away with less as long as it is a quality unit. I would never though go any less than 650. You never know what you might add later, and since the PSU is one thing that will last for years and carry over from build to build, and basically the thing that runs the entire system, never ever skimp or compromise on this item. Its better to have too much than not enough. So unless your budget just demands you trim some money somewhere, and you do not think you would ever add a second GPU, go with a high rated 650 watt unit like this one: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Modern PCs consume much less power than most people think. Read this article at SPCR: http://www.silentpcreview.com/article1245-page1.html
Kevin is right that a good quality 450W PSU could handle your rig. A CF 6850 system with a highly overclocked i7 965 CPU, consumes under 400W: http://www.guru3d.com/article/radeon-hd-6850-6870-cross...
and the 7850 is less power hungry than the 6850 and your CPU with a slight OC will consume much less power than a highly OCed i7 965.

For a single HD7850, you need a good quality 450W PSU with 28A on the 12V rail and (1) 6-pin PCIe connector. For a system with (2) HD 7850 in CF you need a good quality 650W PSU with at least 40A on the 12V rails and (2) 6-pin PCIe connectors. Amps on the 12V rail are more important than the watts; there are 520-550W PSUs that meet the amperage requirements for a CF 7850 setup.

i'd get the 750. PSU have come down a lot in price and you can get a 750~850w for $90 from a quality brand like termaltake, if someday you want to Xfire you will need that PSU.

the rule of PSU vs watts draw:

1) you need the exact amount of watts your PC draws
2) but bad build quality means you need a margin of error
3) the better the PSU, the less the margin

(the above works for both wattage and 12v rails)

the thing is that if you wont crossfire soon he probably wont do it later. for example if i brought a gtx 295 2 years ago i wouldnt buy another one because there will be much better cands now and consume way less power

yes but you need a high quality psu because the wattage doesnt matter. the thing that matters is the amperage on the 12v rails. so if you get a unknown company with like 2 less amps than the standard high quality psu the psu wont really be 750w for example

sure, a 450 watt PSU can handle a rig that consumes 400W e.g but @ what eficiency? an 750W quality PSU with an 50% load (375w) will be more eficient then a 450W PSU and ofc it will generate less heat.

actuarly the capacitors get used out every some time and the power supply will get less efficient and get less amps passed through so i would change my psu every 3 years

agreed but we arenet talking about the temps of the psu or the efficiency we are talking about the amps on the +12 volt rail and sure it would be better if you get a gold or platinium psu. the antec 500w earthwats platinium would be a great choice for this rig since its cheap and very efficiend and also high quality

To be clear the 400W power consumption was on a system with crossfire HD 6850s. A single 7850 will consume significantly less power.

Whose rule is that ?

1) When ? at idle ? while surfing the web ? gaming ? running a power virus to check OC stability ?
You need more than the exact amount in any one of those situations to keep the psu from running at 100% capacity
2) Bad build quality is bad build quality, can't make up for it by going for more watts
3) I agree, a better quality psu can handle a higher load without being overly stressed
Many suggest 50% more power than the max that the system could draw, that tends to blow low level efficiency out of the water, running a high quality psu at 70 or 80% load for a few hours a day while gaming won't hurt it.

Cheap capacitors derate faster.
Quality solid Japanese caps (like Rubycon, Hitachi, Panasonic, Nippon Chemi-Con, Nichicon) can last a decade or more, assuming their temperature or ripple specs aren't
exceeded.

Taiwanese and Chinese capacitors vary more in quality, but most budget units from respectable companies will have caps that can last at least five years. Teapo, SamXon, and OST are reasonably reliable in PSUs.

Ultra-cheap-os will use garbage capacitors like CapXon, Fuhjjyu, BH, LCZ, that have exaggerated specs, and shorter lifetimes.

No need to replace a high quality psu every 3 years, unless you're running it full on 24/7

A quality PSU will last 10 years or more for most users. If you need to replace your PSU every three years, you need to start buying better hardware.