The difference is around 5-10% memory bandwidth in games that depends on memory. If you have a powerful machine that doesn't have the memory as it's bottleneck, you'll never see the difference. However, it is MUCH more stable and easier with 2X1GB, especially at 250. As Dan just said, 200 is a piece of cake but 200 at 2T and 250 at 1T is quite a bit of difference at the same cpu speed. On some mainboards, you can gain if you use 4 sticks actually by enabling true bank interleave, but very few have that option. I would choose 2X1GB any day of the week. At least for 250+.