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ReadyBoost

ReadyBoost lets users use a removable flash memory device, such as USB memory keys, flash memory cards and other types of flash devices to improve system performance by caching disk reads onto the device. This can improve system performance because the Windows memory manager can retrieve data kept on the flash memory device more quickly than it can retrieve data kept on the hard disk, decreasing the time you need to wait for your PC to respond. ReadyBoost technology is reliable and provides protection of the data stored on your device. You can remove the flash memory device at any time without any loss of data or negative impact to the system. Nothing is stored on the flash memory device that isn't also on the hard disk (all pages on the device are backed by a page on disk. No exceptions), so if you remove the device, the memory manager sees the change and automatically goes to the hard disk and your performance returns to the level you experienced without the flash memory device. Additionally, data on the flash memory device is encrypted using using AES-128 to help prevent inappropriate access to data when the device is removed. Also, the memory manager compresses the pages before writing them into the cache on the flash memory device, which means you'll get more mileage from each MB.

ReadyBoost Minimum Flash Memory Device Requirements:
256 MB USB 2.0 or PCI Flash Memory Device
2.5 MB/sec throughput for 4K random reads
1.75 MB/sec throughput for 512K random writes

ReadyBoost Maximum Flash Memory Device Requirements:
1 Flash Memory Device
4 GB Flash Memory Device

ReadyBoost Recommended Flash Memory Device Requirements:
1:1 ratio of flash memory to system memory on low memory systems
2.5:1 ratio of flash memory to system memory on high memory systems (Higher than that and you won't see much benefit)

Notes - "When you connect a supported flash device to your system and choose the Speed Up My System option, Windows Vista runs a quick performance test to see if the device meets minimum standards required for ReadyBoost. These results must be consistent across the entire device. If any of these tests fail, the drive is rejected. You can look up the specific performance results for yourself. Open Event Viewer (Eventvwr.msc) and click the Applications And Services Logs category in the console tree on the left. Under this heading, click Microsoft, Windows, and ReadyBoost. Under this latter heading, select Operational. The log entries in the center pane include performance test results for both successful and unsuccessful attempts