You needn't increase your pagefile really, as Readyboost in no way serves as RAM. It rather caches programs data to a stick instead of the hard drive. So, if your programs do much caching and f your stick/card is much faster than your hard drive, then you can get a significant boost.
Again, in Readybost the flash memory device serves as an additional cache—that is, cache that the computer can access much more quickly than it can access data on the hard drive. But the flash memory IS NOT ram!