depending on how old it is and also if you take the ram out and google some of the number on the label of the ram you should be able to find out if it is ddr2 or ddr3
What RAM you have will depend on your motherboard. For most of LGA775's life, that was DDR2. Towards the end of that socket's lifespan, DDR3 RAM came out, and the last generation of boards, based on the P35 chipset I believe, did support DDR3. What CPU you have is actually irrelevant as back then the memory controller was on the motherboard, and not the CPU, so you'd have to look up your motherboard model and see what type of RAM it took.
For most of LGA775's life, that was DDR2. Towards the end of that socket's lifespan, DDR3 RAM came out, and the last generation of boards, based on the P35 chipset I believe, did support DDR3.