If it is affordable and practical, the first step in tuning a typical system should be adding RAM. After that, ReadyBoost can relieve pressure on the hard drive. ReadyBoost particularly benefits systems with limited RAM and slow hard drives. Any system with over-worked hard drives (queuing, swapping) will likely benefit, particularly older systems with limited RAM. Laptops with hard drives typically benefit, and a low-profile CF or SD card may be a more practical option than a bulky USB drive.
While the smallest cache size is 256MB, larger is better. Sizes larger than RAM can be beneficial, so 1:1 for RAM sizing is not a useful rule of thumb for sizing.
ReadyBoost requires 2.5MB/sec for reads and 1.75MB/sec for writes. Typical USB setup is exFAT with 4k sizing.
ReadyBoost was introduced in Windows Vista for one flash drive. The flash drive capacity was increased in Windows 7. Windows 8.1 allows for multiple ReadyBoost drives.
After ReadyBoost is running, it can be monitored with Perfomance Monitor. Press Windows+X, choose Run, type perfmon
Press the red X to stop monitoring.