is 200/599 2d settings????
there's a difference between 2d settings and 3d settings
2d settings are clocked lower b/c they aren't very demanding (i.e. browsing the internet or typing a word document)
3d is the only thing you'll need to worry about, because that's when it all matters (i.e. benchmarks and games)
one other thing that might limit your OC is your cooling (make sure you card is at an average temperature [see below] and that your psu is adequate)
i'd suggest coolbits2 (can be found at www.guru3d.com
), since you can see your temperatures and detect optimal frequencies that way