Currently I own a silverstone ST30NF which is only 300W.

It can support the following system:
Q9450
p5k Deluxe
Corsair 4gb ddr2
4850 1GB
200GB hdd

I am planning on adding another 4850 in my system hence the need of upgrading my PSU. Will the Seasonic X-400 be sufficient?

Thanks

I have an X-750 and not an X-400 but, as math isn't such an opinion, I'd take the liberty to advice you about.

A couple of 4850 in crossFireX should draw from the PSU around 270W DC under gaming, and around 350W DC under OCCT/FurMark.
Therefore to play safe IMO neither your NightJar, nor the X-400 should suffice.

But even if they're probably pretty cheap nowadays, from a performance perspective would a pair of 4850 worth over a single 5850 (which will draw only around 110W DC while gaming, and 150W DC under stress testing, so as a single 4850 or so about)?

Just my 0.2c, if you don't mind.

Why not just get an x-650 or x-750. Most of the time they will run without the fan. And when you really need the power, they will be there with it.

An X400 will handle it electrically as it's basically a 750W PSU minus the fan. The question is how hot will it get.

To answer that you'd have to consider

the temperatures in your part of the world
the temperatures in the room where you keep the PC
the temperatures in your case
the case design
the case fans
how you'll use the PC

I feel safe saying if you are inside the a shed at McMurdo Station you'd be fine with a fanless PSU so long as you leave the side off the case and let the heat escape or just use all the parts outside of a case.

I think it's also safe to say that in Thailand in the summer you would have problems unless you have a fully conditioned environment.

In addition to the X-650 recommendation consider the Kingwin Lazer Platinum 550W aka Superflower Golden King 550W. Just don't make the mistake of confusing the Platinum with the Gold Lazer 550W. Depending on where you live you might be able to get a 80 Plus Platinum PSU under the Kingwin or Superflower name and save a few bits of local currency.

Of course in the US the Seasonic X are still cheaper by a big amount. amazon x-650

Apart the rated overload while stress testing that I see, but maybe the question according to me should be: is it really worth, greener, more efficient picking a 4850 CFX along with a new expensive (X-400, X-460, X-750, whichever) PSU in 2011 A.D., when a single HD 5850 should give approximately 20% more power (at least, and just for less than half the wattage) than the above mentioned CFX pair?

Cheers for the replies.

Actually, my brother is upgrading his old 4850 so I thought of taking it for crossfire and I have been wanting to upgrade my ST30NF for a while now as it's at least 5 years old now.

Noted the Kingwin Lazer Platinum 550W aka Superflower Golden King 550W. Unfortunately it is not available in NZ. Do you know where can I get this shipped from?

I just realised there is X-460 which is passive as well after more reading.

I guess over here in NZ it's similar to US where X-560 and X-650 is cheaper than X-460.

Perhaps I should go with one of the semi-passive Seasonic.

Seems to me like that is a no brainer. You can't go wrong with that decision.

Seconded. (X-650 owner, fan activates only in heavy gaming)