I did the math not too long ago for this.  Some conservative numbers:

3x5830 (@ $109/e): 750mh/s
2x6950 (@ $209/e): 660mh/s
1x6990 (@ $700/e): 700mh/s

To me, the sweet spot was 5830s for price per mh/s, but I ultimately got a 5830 and a 6950 for my two computers.