memory efficient set/dictionary

[koara]

What is the best to go about using a large set (or dictionary) that
doesn't fit into main memory? What is Python's (2.5 let's say)
overhead for storing int in the set, and how much for storing int ->
int mapping in the dict?

Please recommend a module that allows persistent set/dict storage +
fast query that best fits my problem, and as lightweight as possible.
For queries, the hit ratio is about 10%. Fast updates would be nice,
but i can rewrite the algo so that the data is static, so update speed
is not critical.

Or am i better off not using Python here? Cheers.