On my 3gb laptop anytime I am doing anything process / memmory intensive and I remember I pop up the task manager and have a look and I have never seen memory usage over 50%.

The only thing I have not tried yet and where readyboost might have an effect no matter what the original ram would be something like Photoshop / Gimp and having a lot of big (byte size) images open and the redraw rate whenever you pop one forward. i.e. reading an image that has been paged out to readyboot compared with one that has been paged out to HD.