Ready boost, as mentioned, gives wildly different results. I tried it with a CF card for about a month and clocked absolutely no difference what so ever in any task.
It is suggested to run it in 2 to 1 for best results. For every 1 of gig of ram have 2 of readyboost. However readyboost only works on up to 4 gig with Vista. (256 with Win7.) Having 3 gig of ram makes it almost a 1:1 so that may be what was limiting the performance for me.
What I heard was that readyboost was used as a supplement to the cache memory that a harddrive would use. Which would kinda mean that files would be saved faster...
It does use it as a cache. But for commonly used files accessed on a HDD, not ones that are being written to the disk. It basically keeps track of what is used a lot, and holds a copy in readyboost.

