How many pics does an SD256 could produce using a Canon SD450 5MP digital camera?

Depending on the settings, at least 100.

With maximum quality (2592 x 1944 SuperFine JPEG), each picture is approx. 2,500 KB.
256/2.5 = 102.4

It all depends on your resolution... For best resolution, calculate about 4 MB per pic, which would give you roughly 65

about 100 at 5MP resolution