It's DDR. DDR is just like any other kind of ram -- it can run much slower than it's designed for if you wanted to. the ~350mhz stuff is for the GF2 GTS (lower end, I think), higher stuff is for higher speed chips, and lower speed stuff is...well....for lower grade stuff
The chip you have is slower because the chip is castrated, the frequency of the memory is slower (than a GF2), and the bus width is lower.