ReadyBoost technology takes advantage of the fact that flash memory offers lower seek times than hard disks. Essentially that means that your system can get to a given location on a flash disk more quickly than it can to a corresponding spot on a hard disk. Hard disks are faster for large sequential reads; flash disks are quicker for small, random reads. When a supported external memory device is available, ReadyBoost caches small chunks in flash memory and is thus able to retrieve those chunks, when needed, more quickly than it could if it relied only on the hard disk.

Because an external memory device can be removed without warning to the system, all data cached via ReadyBoost is encrypted and backed up on the hard disk (as well as being compressed). Encryption ensures that the data can’t be read on another system, and backup enables Windows to revert to the hard disk cache in the event that the ReadyBoost drive is removed.

Windows supports the following form factors for ReadyBoost:

•USB 2.0 flash disks

•Secure Digital (SD) cards

•CompactFlash cards

When you connect a device of one of these types to your system, Windows runs a quick performance test to see if the device meets minimum standards required for ReadyBoost. Those standards are:

•2.5 MB / second throughout for 4 KB random reads

•1.75 MB / second throughout for 512 KB random writes

In addition, the device must have at least 256 MB available for the ReadyBoost cache.

Note: ReadyBoost does not support external card readers. If Windows Explorer shows a volume letter for a drive without media (as it does, for example, for card-reader drives or floppy drives), inserting flash media for that volume letter will not give you a ReadyBoost drive. In addition, Windows Vista does not support multiple ReadyBoost drives. (Microsoft has indicated that multiple-drive support is under consideration for future versions.)

How much boost will you get from ReadyBoost?
As with so many other performance issues, it depends. If your internal memory is well above the amount you actually need, ReadyBoost won’t do much for you. If not, you should definitely see some performance improvement. To use ReadyBoost, follow these steps:

1. Plug a suitable external memory device into your computer. An AutoPlay window similar to the following will appear (it won’t say READYBOOST, unless you’ve already assigned that name to the volume, as we have here):

Picture of AutoPlay dialog box showing ReadyBoost drive

This window appears when you plug a ReadyBoost-compatible memory device into your computer

2. Click Speed up my system. If your system passes an initial ReadyBoost test, the Properties dialog box will appear, with the ReadyBoost tab selected:

ReadyBoost properties

Use the slider to set aside space on your memory device for ReadyBoost

•Select Use this device, and then adjust the slider to specify the amount of space you want to use for ReadyBoost. Then click OK.

How much of the external memory device you want to assign to ReadyBoost will depend on whether you also want to use the device for ordinary storage. Microsoft estimates that you can benefit from a ReadyBoost cache equal to approximately 150 percent of your system RAM—for example, a 1.5 GB ReadyBoost cache on a 1 GB systems.

Reference info:
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http://windowshelp.microsoft.com/Windows/en-US/help/596fb57f-cc9d-4ac5-a813-5c0830e9156a1033.mspx
Windows Vista Inside Out by Ed Bott, Carl Siechert, and Craig Stinson (Windows Vista Inside Out © 2007 Microsoft Corporation. To learn more about this book, visit the Microsoft Learning website.)