Oh by the way if I buy me a 5870 will it be bottlenecked by my 720 X3 I got it at 3.6ghz.
There's more CPU overhead for Crossfire than for a single card, so your bottleneck is likely bigger on an Xfire setup.
Are there any facts that a single 5870 will beat 2 4870s?
No facts, but early leaked benchies show the HD5870 usually besting an HD4870X2, GTX295, and two HD4890 in Xfire overall at the higher settings.
BTW you still haven't said why you need one or the Xfire, so what is it, other than an ego boost?