from what I gather, The ready boost can help you alot if you would normally depend on paging files...(meaning you don't have much ram)
*(paging file is an area on the Hard Disk that Windows uses as if it is RAM)
*The hard drive is much slower to read from than a high speed flash drive

so say your comp has only 512 MB Ram; using ready boost would be much faster than using paging files on the hard drive to edit movies or whatever other high resource programs you run.

I have noticed that it seems to slow my comp down if I try using ready boost because 2 GB of ram takes care of most every day tasks and I don't usually need to use page files.