It appears that you really will not lose any performance in choosing a 915 motherboard with DDR support instead of 915 with DDR2. With initial launch prices of DDR2 still considerably higher than DDR prices, you can choose a 915 board with DDR and not have to worry about losing any performance.
As prices for DDR2 drop more in line with DDR, the choice becomes a little more interesting. DDR2 will mature beyond the current 4-4-4 timings, and as timings and headroom improve, DDR2 becomes a more interesting choice for memory.