I've tried up to 3 grams at once. I didn't really notice much difference between 800-900mg and 3 grams, honestly. Keep in mind I use it in bulk crystal form - some reports claim that nootropil and other forms of piracetam in pills seem to be less effective, and the times I've tried Nootropil in particular, I've found this to be true. Plus, it's WAY, WAY cheaper if you buy it in bulk.
Anyway, I've found the most benefit from piracetam by taking it every morning at 800mg for a long period of time. I notice it more each day, and I've been taking it for a few months now. Also, combining it with hydergine and centrophenoxine has helped its benefits a lot and reduced the amount I need (which is the reason I now take 800mg daily instead of around 1500).
EDIT: Oh, I didn't realize you were talking about pramiracetam. Nope, never tried any of the other 'racetams.