I think you're putting the cart before the horse.
While upping the core frequency will add to the framerate, it isn't as significant as upping the memory frequency.
Set the core back to default (175) and then up your mem setting, try 450, then 475, then 500, then just keep going! I found the best way to test was to run the first benchmark in 3dmark2001. It tells you more or less straight away if you've pushed the card too hard as artifacts start appearing.
Once you've reached the ceiling for the mem, push the core up to 200 - you could go higher, but make sure that the card's stable.
I found attaching a better fan to the gpu chip helps when pushing the core speed up - I got a gf2pro up to 245.
hth.