RAMBUS only needs to be installed in pairs where the chipset implements a dual channel architecture. This applies to the i840 and i850 chipset, but not the i820 chipset. Therefore RDRAM does not need to be installed in matching pairs on the i820 chipset.
The dual channel implementation doubles bandwidth to the chipset, as the chipset can access both channels at once, increasing performance. Think of two jugs pouring into a glass. Two jugs will give you twice as much, but in order to keep a balance, you need to pour both at the same rate.
SIMMs, or Single Inline Memory Modules, provided a 32-bit data path to memory. This was fine for the 486 processor, since its data bus was 32-bits wide. FPM and EDO RAM did not need to be installed in pairs on 486 systems.
SIMMs needed to be installed in matching pairs on the Pentium processor because the data path was 64-bits wide. A pair of 32-bit SIMMs were multiplexed into a single 64-bit wide channel, matching that to the processor.
DIMMs provide a 64-bit data path. This is why you can install DIMMs singly on Pentium and later processors.