I would look for a bargain 7970 in all honesty - I have tried just about everything else.

One 7970 gives more mh/s when overclocked than 2 overclocked 7950's

Overclocked they will do 700mh/s

2 graphic cards in a pc case is bad news - one is usually pushed up against the side so has no airflow and runs hot - ejecting its hot air straight into the intake of the next card.

Honestly the cooling is so badly designed for multi card setups.

Do you plan on doing anything like gaming?