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.
ditto aevm i have a Q6600@ 3.6 and 4870 X2 and i was going to get a second one and relizied proformace was bad horrible scaling and somtimes even LOST PROFORMACE also good card to get for half the price of 4870 x2 is a 4850 X2 competes with GTX 280 and is only 224.99$$$!!! http://www.newegg.com/Product/Product.aspx?Item=N82E168... i would of got this if it came out sooner but i was so focused on a 4870 X2.... but what ever you choose it will be a good option