I have been constructing a new system on pcpartpicker.com and I've got an estimated wattage of: 394W (http://pcpartpicker.com/p/2kK5b)
So, is it ok if I got myself a 400W PSU or do I need a stronger PSU for this build?
(Note: I'm not going to upgrade any of the parts or add another graphics card)
Also what is the difference between active and passive PSU?
Is there anything else I should watch out when looking for PSU?
I would get at least a 430-450W PSU. A 500W would be the best.
What is the difference between active and passive PSU?
400W should be fine, but that depends on if the unit can actually provide 400W. That Corsair unit is okay, but I'd personally go with a Seasonic G 550 for a higher efficiency, better build quality, and generally lower ripple. If that TX is an older series, it is a CWT OEM, which can be pretty (and usually is) low-end, but if it is the newer model, then it should be a Seasonic OEM. If it is the older model of the TX at that price, then switch to the G 550. If it is the new model, that is fine. Regardless, though, 400W is plenty.
Active most likely refers to the cooling method, with a fan. Passive would therefore mean there is no fan, and it is cooled via a sink.
When buying PSUs, to answer your last question, do not pay attention to brands - look at OEMs, and specific OEM models. You want a high efficiency, a low ripple, and enough connectivity for whatever you are working with. Wattage-wise, most people over-estimate; 400W is plenty for a single GPU + CPU + HDD. Just be reasonable, read reviews that aren't on Newegg and Amazon, and try to get a dissection of the PSU if possible.
Active or Passive PFC you mean? Active PFC is more efficient, and anyway I think most decent PSU's nowadays are Active PFC (Power Factor Correction). PFC deals with the conversion of AC power from your wall to DC current that your PC uses. I don't know all the technical details about it though.
OEM is original equipment manufacturer? What OEM should I look out for?
Wait... When talking about active or passive, were you talking about the cooling or PFC? I have some good articles if you want to learn about PFC.
I'm clueless about both, actually I'm clueless about PSU in general
Jonnyguru.com's reviews are a great resource to learn how PSU's are evaluated.
Engineers select parts by viewing the specifications carefully, and how they perform under what conditions. A good overview of specifications for many different PSUs can be found in Corsair's chart, which is at the top of this section of the forum.
PSU's are rated at a certain temperature and if you want them to give up the Watts, you have to keep them below that temperature. In small cases, or with cheap PSUs this can be important.
There are important voltage regulation circuitry on your motherboard as well.
I would be looking into something with some headroom. Under full load, the 400w unit's fan will be going nuts and running at close to capacity will shorten the units life.
Going cheap on low wattage and low quality PSUs always come back to haunt people.
The moment you oc either the cpu or GPU with a 400w'ish PSU shit will hit the fan.
Stick with the 650w corsair unit. Or even switch to a xfx unit.
Actually, passive PFC has better efficiency than active PFC, but it can't handle as much power as the active PFC. Quote: "On the other hand passive PFC has higher efficiency compared to active PFC!" http://www.techpowerup.com/articles//overclocking/psu/160/4
The above link is a great, very technical, introduction into the world of PSUs.