RDRAM

Rambus Dynamic Random Access Memory: This is a type of synchronous DRAM created by the Rambus Corporation. RDRAM features an architecture designed to achieve high bandwidth, it is used in the Sony PlayStation 2, early Pentium 4 desktop systems and other applications. The XDR DRAM, RDRAM's successor, is used in IBM's Cell processor and Sony PlayStation 3. RDRAM is also mainly used for capacity expansion of old desktop systems and often come in the form of 184-pin RIMMs/Rambus Inline Memory Modules (16bit). 

SDRAM

Synchronous Dynamic Random Access Memory: SDRAM has a synchronous interface. It waits for a clock pulse before transferring data and is therefore synchronous with the computer system bus and processor. This greatly improves performance over asynchronous DRAM. SDRAM is not as popular as it once was and may be used during upgrades. SDRAM modules usually come in the form of 168-pin DIMMs.