If you decide to Xfire, you will need to get a better PSU, the one you have will not handle it.
Hell I would replace the PSU anyway, its a VERY bad unit and will likely fail at some stage or another.
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=120
Now the PSU you chose to replace it with is a little overkill. There are much cheaper options in general if you are willing to live without modular. But that PSU is VERY good.
So my suggestion :
Get another HD7950 and get a new PSU.