The main difference between the 600 series and the 400 series is added support for DDR333 memory. Since both the 620-D and 615-D are dual channel solutions, DDR333 support increases their peak theoretical memory bandwidth to an impressive 5.33GB/s, a 25% improvement over the previous 420-D. What's interesting to note is that there is no mention of a cheaper single channel solution like the 220-D. If anything, the 615-D should be a single channel solution as we've already proved that having dual 64-bit memory channels does not improve performance without an integrated GPU to use the extra memory bandwidth.