vegas311 :
Yea I have 2gb boost,. And it's 2 months old. And I've read a lot of bad reviews with xfire.
Set aside the money, if I'll get 2 r9 270x is it a better choice than 2 gtx 650 ti boost?
ok. lets erase what you know about xfire for a moment. It will be much easier without preconceptions and prejudices, because as you know things change in the computer industry weekly.
1) both xfire and SLi are increadibly buggy and don't always work as they should. Often times games will crash or stop working right. So the general advice is "get a single gpu over a xfire/sli setup whenever possible"
2) xfire works as good as SLi with dx11/11.1/11.2 titles AND a single monitor
3) xfire does not work as good (or at all really) as SLi in dx9 or with multiple monitors (with one exception, xfire works fine with dx9 and multiple monitors if you're using an r9-290/r9-290x gpu)
4) because xfire does not work as good as SLi in dx9 or with multiple monitors that does not mean SLi is flawless in those situations. SLi remains buggy in it's own right, and has it's own struggles with both dx9 and multiple monitors.
5) AMD is fixing xfire currently. and the situation will likely change in the future. Currently xfire scales far better then SLi, and in the situations it works, it's indistinguishable and works as well as SLi. The r9-290/290x actually work better in xfire then any nvidia SLi configuration, proving AMD can get it right when they put their mind to it.
So... to answer your question, currently SLi'ed 650ti BOOSTs would be the better all round option. However, when AMD gets the kinks ironed out of xfire in dx9 titles and multiple monitor situations expect the r9-270x to be a significantly better performing combination for 2 reasons. 1) the gpus themselves are about 20% faster. 2) xfire scales significantly better then SLi further widening the difference in performance.
i speak about scaling a few times.... what i mean is two cards in xfire/sli aren't as strong as if you just added their performance together... there is performance loss so it's not as simple as adding 1+1 = 2. 2 cards in SLi typically give you around 150%-175% the performance of 1 card. 2 cards in xfire typically give you 185%-195% the performance of 1 card. So we say xfire (when it works properly) scales better then SLi.
Previously, because of the issues with xfire, the issues with SLi scaling weren't really a concern, because SLi gave the end user a better experience. Now that AMD is finally addressing the issues with xfire the focus shifts to the poor scaling in SLi nvidia gets (something which is really obvious when you look at how 2 r9-280x/7970s in xfire perform about as well as 2 780s in SLi).