I agree, kind of.
The second card will add very little value because games don't scale well beyond 2 GPUs. So let's take out that second card and discuss the CPU now.
A system with a HD 4870 X2 will do almost exactly the same in games whether it has a Q6600, a (faster) Phenom II 940, or an (even faster) Q9650 overclocked to the max.http://www.legionhardware.com/document.php?id=770
So, I guess, the answer is that the CPU will not be a bottleneck, but the second card is pointless. OK, there are exceptions, i.e. exceptionally well programmed games like CoD which scale very well to 4 GPUs, but normally even the third GPU is almost useless.
Also, two HD 4870 X2 cards would need a $250 1KW PSU. With a single HD 4870X2 you can get by nicely with a $80 S75CF 750W.