Yes, you can. If you have a motherboard that runs at 333 (ddr333), you can use ddr400 (pc3200). The memory will just run at the slower speed. Also, you always want the front side bus (FSB) to be in sync with the memory speed. For instance, if you have a processor that runs at 166 x2 (333), on many boards you can set the memory to either 166 or 200 (ddr333 or ddr400). But for best performance the speed should always match, 166 fsb, 166 memory.