Python dictionary size/entry limit?

[intelliminer at gmail.com]

I wrote a script to process textual data and extract phrases from
them, storing these phrases in a dictionary. It encounters a
MemoryError when there are about 11.18M keys in the dictionary, and
the size is about 1.5GB. I tried multiple times, and the error occurs
everytime at exactly the same place (with the same number of keys in
the dict). I then split the dictionary into two using a simple
algorithm:

if str[0]<='m':
    dict=dict1
else:
    dict=dict2

#use dict...

And it worked fine. The total size of the two dictionaries well
exceeded 2GB yet no MemoryError occured.

I have 1GB of pysical memory and 3GB in pagefile. Is there a limit to
the size or number of entries that a single dictionary can possess? By
searching on the web I can't find a clue why this problem occurs.