No. Readyboost will only use up to 4gb of a device. Through compression, readyboost effectively doubles the amount of space for readyboost, however. for example, 2gb given to readyboost can contain approximately 4gb of data.
Also, readyboost isn't "ram"... it's a place for the system to cache files. flash memory is generally faster for this than your hard drive, so you see an increase in speed. though i believe that if you have a lot of actual ram, you don't see much performance increase.
you can read about it more on wikipedia:
ReadyBoost - Wikipedia, the free encyclopedia
and on a blog by some MSDN guy:
Tom Archer's Blog : ReadyBoost Q&A