i have a evga gtx 780 sc acx gpu the effective memory clock is 6000 but when i look on gpuz it says 1500
That is correct. The GPU memory uses GDDR 5, which has an 'effective speed' which is 4X it's frequency (compared to original DDR ram).
So on that basis 1500mhz GGDR5 ram = 4 x speed of 1500mhz DDR = 6000
It's basically a marketing trick to make it sound better.