GDDR3 memory works in something called double data rate. that means that it can effectively do 2 operations in 1 cycle, so the 1400Mhz is the effective number. the number that you would see in a program would be half that, 700Mhz. i'm not sure where you got that 829Mhz, unless you overclocked it.
Thanks or clearing things up. Helpful!~