new overclocking 8800 gts 320mb

I get about the same results. If you dont mind me asking, what volts are you at on your cpu? Are you using watercooling? Temps?

The setting 600/875 is point less. Either go 594/864 or 612/891(900)

The core clock and the memory clock go up in certain step, even if you set it at 600 it will either go down to 594 or go up to 612, it's usually go up. So it's better to know what step you are in and what's the next step is.
Core clock steps:
567, 576, 594, 612, 621, 648, 675, 684, 702
Memory steps:
783, 792, 810, 828, 837, 864, 891, 900, 918,936,945, 972, 999, 1008.