There have been a few studies on Readyboost. the general conclusion is for less than 2 Gigs of RAM it shows some improvement. 2 Gigs and above the improvement is minimal.
It is cheaper than RAM of course but getting extra RAM will be a better route.
Two other points. Readyboost is not the same as extra RAM, it's more of a cache and is not as effective as RAM. Secondly, not all flash drives or at least the flash units like compact flash cards used in cameras and the like are readyboost optimised. readyboost needs fast read/write ALL of the time rather than for single or small groups of instances. If you buy units get those which are marked Readyboost ready/optimised or similar.