You need RAM that will run at PC3200 speed (200 MHz = 400 MHz efffective after DDR)
That doesn't necessarily mean it needs to be PC3200, as I have 2 sticks of PC2700 that run well at PC3200 speed.
FSB = Front Side Bus, which is (in simple terms) how fast the processor communicates with the memory controller.
DDR = Double Data Rate. What this really means is that the effective frequency is double that of the actual frequency because somehow (don't ask me to explian how) the RAM is able to transfer data at the low AND high points of a waveform. Imagine a sine wave... one wavelength is a peak and a valley. SDRAM would transmit once per wavelength, DDR RAM would transmit at the peak and the valley, or twice per wavelength.