By the sounds of it your probably going to buy a real cheap AMD processor to compensate for dual 660's which is probably going to result in bottleneck. Anyways state the AMD processor your planning on using. I myself am leaning towards the 3570k and 660, processor is important as well.
Just buy a single one with the i5-3570k, or a i5-3470 + 660 gtx ti/670 gtx/7970.
Oh and with two 660 gtx its called sli setup they're from nvidia not a crossfire setup(amd).
Two 560 ti sli will offer poor value for the money nowadays, the 660 ti gtx performs the same as that setup with less troubles and power consumption.