Some useful information about using ReadyBoost with a USB flash drive:
The drive does not act as extra RAM, or as virtual memory - it is used as a file cache for frequently accessed files. If the system has a slow hard drive, or typical tasks generate a lot of seek activity on the hard drive(s), this may be helpful. If the system is low on RAM, the assistance will be most noticeable, but far more benefit would come from a generous complement of RAM.
Note the minimum performance requirements for a flash drive to be used this way:
At least a 2.5 MB/sec throughput for 4-KB random reads
At least a 1.75 MB/sec throughput for 1-MB random writes
Compare this with hard drive figures, such as here:
Random Read 4KB (QD=1) : 0.578 MB/s
Random Read 4KB (QD=32) : 1.037 MB/s
Random Write 512KB : 47.439 MB/s
Random Read 512KB : 43.852 MB/s
Readyboost has the potential to enhance small random reads significantly (eg intensive access to random database records), but a hard drive is much faster for large reads and writes. So a Readyboost flash drive can help in this limited circumstance, but in many cases with a decently specified system and the usage patterns of a lot of users, will bring little benefit.