The memory on a video card is seperate from the memory on the motherboard and therefore it doesn't matter if the video card has DDR3 and the motherboard has DDR2. You can put the video card with the DDR3 on a motherboard that has DDR2 memory.
Like other says, they are different types of memory. A GDDR5 GPU can even be put on DDR Motherboard. The things that you see is your CPU (whether it will bottleneck or not), PSU (Enough for your GPU or not), The motherboard slot (Make sure you have the correct slot for the GPU) and your Case (space).