DDR2 is not currently really any faster than DDR. DDR2 requires a special motherboard and cannot be used in a motherboard designed for use with DDR.
Unless you purchase a system which needs it, there's not really any point to purchasing it.
When DDR2 was designed, it was not envisaged that DDR modules would get as fast as they have. The DDR2 modules only go a little bit higher in clockspeed, but have much higher latency than fast DDR modules. Balances out a bit.