The RAM used in the Xbox 360 is a single pool of 512MB GDDR3. The chips are clocked at 700MHz and sourced from various vendors.
The initial launch RAM vendors were Samsung and Infineon (Qimonda). Hynix chips became an option starting with Zephyr. Options for Elpida and Winbond became available with the Xbox 360 S. Each vendor updated the chip provided a few times, and some vendors stopped providing chips altogether.
Elpida supplied one 128MB chip variant for the Trinity motherboard.
The 512MB of RAM is shared between the CPU and GPU, with the GPU being the memory controller. Almost all of the RAM is accessible to games as the Xbox System Software is designed to have a very small memory footprint.
Xenos supports multiple RAM configurations from 128MB to 1GB.
128MB Chip Support
On Xenon and Zephyr, only 32MB and 64MB chips are supported. Starting with Falcon, traces were added from chip select 1 on the GPU to the top in addition to chip select 0. This allows 128MB chips to work, enabling the 512_4 chip configuration.
Standard systems configured as a development kit had the normal amount of 512MB. In 2010, the XDK-GB was released with 1GB of RAM. Previous motherboards did not support 1GB due to the 13th addressing line to each chip being not connected. This was added along with other minor changes for the XDK-GB. This change was never made on motherboards that were not destined to have 1GB of RAM installed.
The RAM operates as 2 separate pools. The first 512MB is used the same way as a normal system, and the upper 512MB was used for debugging.This ensured that debugging tools did not take up memory that could otherwise be used by a game. The XDK-S based on the Xbox 360 S continued using 1GB RAM.