Due to the system limits, I can only use a 64bit version Python, though, some of the modules that I intend to use do not come with a 64bit version (e.g. httplib2). In this case, what am I supposed to do so as to leverage those 32bit functions?

Thanks for the answer by j_mcnally. That works. Another approach that eventually works for me is to use the prefix parameter if there are multi-versions(e.g. 32/64) of python installed:
python setup.py install –prefix = “your python root directory”
Excuse my ignorance, but can’t you compile them as 64 bit modules?
http://code.google.com/p/httplib2/
the source is there. I would image most modern ./configure scripts will detect your arch, and compile the appropriate extension.
the following seems to work for me:
ARCHFLAGS='-arch x86_64' python setup.py install