Well ... I suppose one solution would be to ditch the 4600 and hook all 3 monitors up to the 6950. You can use all 3 without SLS (single large surface ... continuous desktop) mode if you prefer. This would cut down on power, heat, and hassle.
All you need is an active adapter for the displayport to whatever cable you have going to the third monitor (or a displayport cable if your monitor supports it natively)