ALTMEMPHY settings "memory clock frequency"

Double data rate memories don't double the clock rate, they just use two samples during each clock cycle which effectively doubles the data rate.

So if you use 333MHz memory (DDR667) then everything is clocked at 333MHz unless you choose the half rate controller which means the user (local) interface will operate at 167MHz while the PHY still operates at 333MHz.

So your example of DDR2-800 means that the I/O and PHY operate at a 400MHz clock frequency. The local interface will operate in half-rate mode so that brings the clock frequency of the local interface down to 200MHz. So in terms of bandwidth everything is matched. If the memory device itself is 16-bit in width at DDR2-800 speeds, then the half-rate local side will be 64-bit and operate at 200MHz. To answer your question you would type 400MHz into the clock frequency edit box.