Trying to decide what to order for my new build (I5 2500K build)

Seasonic X-760 or CP-850? (unless anyone has a better advice then these 2)

I think the seasonic should be close to dead silent in my P183 build or should I get the CP-850??

My strong suspicion (without seeing the complete build) is that you don't need anything more than a Seasonic X-560. My personal preference is for the Seasonic X series over the CP-850, partly because the Antec is way over-powered, and also because Seasonic has a history of being more reliable than Antec (although I don't have any reliability info on the Antec, nor do I know who makes the Antec CP-850).

M., I support your comment but one can extrapolate from this.
(Sorry, J., I cannot comment on the two specific ones you are asking about. Maybe another member can. Just some general advice from an experienced builder.)
Any mid range power supply is probably enough, so you should be looking at features such as reliability, warrenty, cable modularity, effieicnecy, and even the paint design if you have a "tie" or a side window.
But more important if you are at SPCR...noise.
For noise reasons, if you buy a P/S over-rated for your system needs, you will never have to use it above base RPM for the Fan (I use 50% max load on the P/S as a personal guideline). What I did on my last build was retrofit the fan in an old P/S at 2X needed power rating with a nectua 120, down volted the fan, and I can't hear it ever. Newer P/S have better fan control, so may well be quiet enough without fan or voltage change.

Read the great build articles from late last year, they discuss P/S choices, at http://www.silentpcreview.com/Silent_Home_Server_Build_Guide

I Like your choice of CPU for you build, this what I will use for my build # 12, for my wife.

Delta builds the CP-series for Antec, thought Seasonic also builds PSUs for Antec (old Earthwatts).

The advantage of the CP-850 is that it's really cheap for the wattage and really quiet. But I think The seasonic x-series is likely one of the few that can rival the CP in quietness, while having higher efficiency, but I assume it'll cost more. Of course it's also standard sized which can be both an advantage and disadvantage. I think it depends on your priorities, but personally I'd want the X-series.

Mind you, the recent puget i7 2600k sandy bridge system is built with a p183 case plus antec cp-850 psu. It gets like 14 db or something. Near dead-silent. I don't know whether the seasonic x series will be able to battle with that silence. Maybe when the fan isnt engaged,... however how on earth should one build a seasonic x series psu in a p183??? Wont it build up heat and wont the fan face the wrong direction?

Also, although the seasonic has a fanless operating mode, it does come equiped with a fan. Im not sure when itll engage, i thought i heard a review video somewhere say itll be engaged at 25% load of the power. After that itll add 16db when loaded up untill 50% of psu power. After that, itll ramp up enough so as to produce very noticable sound.

Your opinions on the

seasonic x series versus cp850 silence when build in the p183?

I have yet to hear my Seasonic X or Corsair AX. The reason is that if you really have something eating up 300+ watts, then usually the system is already louder than the maximum sound of power supply.