That is correct... You could not fit DDR into DDR2. Both have a different pin count.
XMS (Extreme Memory Speed) is Corsair's brand of high performing memory. XMS can currently reach up to 500 Mhz (400 Mhz (PC3200) officialy), and it is for the DDR DIMM (184).
XMS2 (Extreme Memory Speed 2) is Corsair's new line of permorming memory for the DDR2 DIMM (240). At the moment, it can reach upto 667 Mhz (PC5400), however, it has the potential to reach much higher speeds. The high frequency makes up for the CAS latency timings.