RDRAM
SDRAM
SRAM
DRAM
That's not really correct John. RDRAM, SDRAM, etc. are all types of DRAM.
SRAM is a type of RAM used for level 1, 2, and 3 caches on or around microprocessors. SRAM is a hell of a lot faster than DRAM. The disadvantage is that SRAM is very very expensive, which is why it is perfect to use in cache architecture. Level 1, 2, and 3 cache architectures usually use no more than 1MB of cache (SRAM), whereas RDRAM and SDRAM can be found in sizes of 1GB or more (DRAM). DRAM is much cheaper than SRAM, which is why it is used for RDRAM and SDRAM modules.
This is the correct order:
SRAM
DRAM (which includes RDRAM and SDRAM).