Windows Ready boost..what is it?

Using ReadyBoost-capable flash memory (NAND memory devices) for caching allows Windows Vista to service random disk reads with performance that is typically 80-100 times faster than random reads from traditional hard drives. This caching is applied to all disk content, not just the page file or system DLLs. Flash devices are typically slower than the hard drive for sequential I/O, so to maximize performance, ReadyBoost includes logic to recognize large, sequential read requests and then allows these requests to be serviced by the hard drive

When a compatible device is plugged in, the Windows AutoPlay dialog offers an additional option to use the flash drive to speed up the system; an additional "ReadyBoost" tab is added to the drive's properties dialog where the amount of space to be used can be configured 250 MB to 4 GB of flash memory can be assigned. ReadyBoost encrypts, with AES-128, and compresses all data that is placed on the flash device; Microsoft has stated that a 2:1 compression ratio is typical, so that a 4 GB cache could contain upwards of 8 GB of data

According to Jim Allchin, for future releases of Windows, ReadyBoost will be able to use spare RAM on other networked Windows Vista PCs

For a device to be compatible and useful it must conform to the following requirements:
The capacity of the removable media must be at least 256 MB (250 after formatting)
Devices larger than 4 GB will have only 4 GB used for ReadyBoost
The device should have an access time of 1ms or less
The device must be capable of 2.5 MB/s read speeds for 4 KB random reads spread uniformly across the entire device and 1.75 MB/s write speeds for 512 KB random writes spread uniformly across the device

The device must have at least 235 MB of free space
NTFS, FAT16 and FAT32 are supported
The initial release of ReadyBoost supports one device
The recommended amount of memory to use for Windows ReadyBoost acceleration is one to three times the amount of random access memory (RAM) installed in your computer

Performance
A system with 512 MB of RAM (the minimum for Windows Vista) can see significant gains from ReadyBoost. In one test case speeding up an operation from 11.7 seconds to 2 seconds (increasing physical memory from 512 MB to 1GB reduced it to 0.8 seconds). Systems with 1 GB or more do not show a significant effect on tests to date.
The core idea of ReadyBoost is that a flash drive has a much faster seek time (less than 1 millisecond), allowing it to satisfy the requests fairly quickly compared to a hard drive when booting or reading certain system files. It also leverages the inherent advantage of having two parallel sources from which to read data. Unfortunately, low-cost flash drives are slow in terms of sequential reads and writes, compared to modern desktop hard drives -- 7200 rpm hard drives can sustain 60-80 MB/s, which is 6 to 8 times faster than the 10 MB/s sustained by the fastest low-cost flash drivesThe only advantages these flash drives have are a seek time of around 1ms, compared to the 8-12ms typical on modern SATA drives.High-cost ($1,000-$50,000) solid state memories currently have random sustained external throughput up to 3 GB/s (TMS RamSan) and latency as low as 0.003 ms (Violin 1010)

On laptop computers the performance shifts more in the favor of flash memory, laptop memory being priced relatively higher than that for desktop systems, and with many laptops using relatively slow 4200 rpm and 5400 rpm hard drives. Additionally, on a laptop, the ReadyBoost caching can reduce hard drive access, allowing the hard drive to spin down for increased battery life Additionally, because of the nature of the power management typically enabled during mobile use of a laptop it is a more power efficient way of increasing equipment productivity.
The performance of NAND flash caching (in the form of Intel's Turbo Memory technology) has also been called into question by some computer manufacturers