For the Ti200 that sounds about right. 4ns = 250Mhz or 500Mhz DDR. You should be able to overclock your Ti200 to 500Mhz memory without going out of spec. I think the default is 460Mhz..That could be for the plain GF3 though. Someone correct me.
The Ti500's use 3.8ns memory which is spec'd out at 526Mhz DDR. I'm running mine comfortably at 550Mhz.