That's the basis, but with more inconsistent performance numbers. Some games will see a large boost (<80%) from two parallel cards, others will see will see smaller boosts; some games may even show no impact at all or a negative impact. Overall a very erratic behavior.
9600GT has a boost of around 80% average if I am correct. 8800GT is lower then that... But still 8800GT's SLI will beat 9600GT SLI because of raw power.