There are certain LGA775 boards that allow you to use DDR and DDR2 on the same board (not at the same time). Those boards allow you to upgrade as needed. There are some LGA775 boards, mostly based on 910 and 915 chipsets that allow you to use DDR alone. All LGA775 boards based on 925x and 925XE chipsets and some on 915 allow you to only use DDR2. There are also some LGA775 boards on the 865 chipset that allow you to only use DDR memory. What is your board model? Yes it does make a difference. Right now DDR and DDR2 perform equally, but DDR2 will be the next generation memory.
