That's a Rambus DRAM (RDRAM) Rambus Inline Memory Module (RIMM). These were 184-pin memory modules that came in capacities of 64MB to 512MB with speed ratings of:
- PC-600 (266MHz clock, 1066MB/s bandwidth)
- PC-700 (355MHz clock, 1420MB/s bandwidth)
- PC-800 (400MHz clock, 1600MB/s bandwidth)
- PC-1066 (533MHz clock, 2133MB/s bandwidth) (for 850E, might work with 850 depending upon chipset and CPU stepping and BIOS)
There were some even faster ratings, but those were the speeds I saw installed in desktop PCs and workstations.
The most common chipsets paired with RDRAM were the Intel 850 and 850E chipsets, which supported Socket 423 (Williamette) and Socket 478 (Northwood). The 850 chipset support 1.5GB of RDRAM, in the form of 2×512MB and 2×256MB pairs, while the 850E chipset supported 2GB (4×512MB).
Motherboard were either made with two or four RIMM sockets, and memory had to be installed in pairs. Unpopulated memory slots had to have electrical bus termination cards installed in them called continuity modules.
RDRAMs were made both with and without ECC, and you had to install the same kind in both pairs of sockets. If you installed two ECC pairs and two non-ECC pairs of RDRAM, ECC would be disabled across all.
For best results, though, you would want to get memory with the same timings and specifications, even if the densities were different (e.g., 512MB and 256MB modules for the 850 chipset).