If you install a single DIMM in an Intel Dual Channel Memory system, you will achieve only half the Memory performance that should be possible. Installing matched pairs of DIMMs – in the correct sockets – will double the Memory Performance and increase the overall system performance by 10-25%, compared to systems with the single DIMM. Background For background information on the Intel Dual Channel Memory Architecture, please see these documents: Intel Dual Channel DDR Memory Architecture White Paper http://www.kingston.com/newtech/MKF_520DDRWhitepaper.pdf Dual Channel DDR400 memory can balance the performance of the Intel Pentium 4 processor with 800MHz front side bus. http://www.intel.com/design/chipsets/dual_ddr_sb.pdf Personal Experience I bought a PC from a major manufacturer that had a single factory-fitted 256MB DIMM. I installed a matched pair of PC3200/DDR400 DIMMs - in the correct sockets - and measured the following memory performance improvement: Pentium 4, 3.2GHz Single DIMM 2947 MBytes / sec Dual DIMMs 4664 MBytes / sec ( SiSoft SANDRA benchmark ) The Intel and Kingston documents suggest that this results in a 10-25% improvement in overall system performance. I did not learn about this possible performance improvement by reading the manufacturers literature, as this aspect was poorly documented, or by calling their Technical Support line. I hope that this helps users understand this subject better.