Have a look at the Ramtron FM24C512.

512Kbit Ferroelectric Nonvolatile RAM
• Organized as 65,536 x 8 bits
• High Endurance 10 Billion Read/Writes
• 45 year Data Retention
• NoDelay™ Writes
• Advanced High-Reliability Ferroelectric Process

Fast Two-wire Serial Interface
• Up to 1 MHz Maximum Bus Frequency
• Supports Legacy Timing for 100 kHz & 400 kHz

http://www.ramtron.com/products/nonv...duct.aspx?id=5