It really depends on the speed of the RAM. A 512Mb SD-RAM running at 400 Mhz is better than a 512Mb DDR-RAM running at 100Mhz. If they were both running at the same speed (say 400 Mhz), I think the DDR would be better though (because it's a newer technology). As mentioned before, DDR does have higher speeds than SD has. But keep in mind, you need to get the type of RAM that your Motherboard supports. This includes...1) RAM type (SD or DDR)2) RAM speedMotherboards that only support up to RAM speeds of 200Mhz, will have problems if you stick in a 400Mhz speed RAM. It's VERY important to make sure you get the right type of RAM for your motherboard! Look at your motherboard manual (or look it up online) and see what kind of RAM it supports BEFORE you buy the RAM. Otherwise, you could be wasting your money.