Vadim, Depending on the nature of your data, CouchDB (http://couchdb.org) might be worth looking into. It speaks JSON natively, and has real-time map/reduce support. The 0.8.0 release is imminent (don't bother with 0.7.2), and the community is active. We're using it for something similar to what you describe, and it's working well.
Chris -- Chris Anderson http://jchris.mfdz.com
