I have been mining now for a month with 2 x 9750 on Slush's pool.

Although the total has rate of the pool has increased and so the difficulty, my reward per day has increased.

Why?

Because the total has rate of the pool has increased, the pools luck and the number of blocks found per day, has increased. Although my hash rate percentage of the pool has decreased, and thus the reward per block found has decreased, I get more rewards per day due to the higher number of blocks found. I am thus getting a higher return per day.

I am not giving up on mining with a GPU. Once I can not keep up with the total pool hash rate, I am going to switch over to alternative coins.

I am not sure about the 8 minutes. The average time to find a block on Slush's pool is 1 hour. This was averaged over a day, over a period of 3 days, with an average of 17 blocks found per day.