Both are top quality units, you can't go wrong with either. The Seasonic X has better load regulation, though the g2 has better ripple.
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=332
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=429
The EVGA g2 unit has 10 year warranty, whereas the X-850 has 5-7 year, depending on the model.
Sli isn't a good idea. I suggest when you need to upgrade, sell your current card and buy a single powerful one. I doubt any new technology will use ~500w from your PSU.
This build really is fine and comfortable with 550w.