Most likely, you'll need a Russian visa to enter Russia. To obtain a visa from Russian consulate, you'll need an invitation which can be obtained from a travel agency. Please make sure you do this in advance, to avoid expensive urgent actions.
The Monomax agency which provides services for EDBT/ICDT'2009 offers a package including visa invitation and accomodation in different hotels with a range of prices. More details are available here.
You can also find useful information on travel to St.-Petersburg and local public transportation the Monomax site (use the link above).
Alternatively, you can obtain visa invitation and accomodation from your travel agency.