Obsolete part — sourcing through independent channels
Cypress Semiconductor's CY7C1568KV18-450BZXI is a 72 Mbit DDR II+ synchronous SRAM organized as 4M x 18 bits, clocked at 450 MHz over a parallel interface.
450 MHz clock — what it buys you
The 450 MHz clock rate is the headline speed for this DDR II+ SRAM. In a typical networking line-card application, that translates to a peak data rate of 900 million transfers per second on the data bus (double data rate). The 4M x 18 organization gives you 72 Mbit of storage, which is enough for deep packet buffers, look-up tables, or large cache structures in telecom infrastructure equipment.
It is not a field-swappable part; you will need a reflow oven and a stencil to mount it.