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