OAIster, a cross-archive search engine, now covers over 200 OAI Archives (about half of them Eprints.org Archives) indexing over 1.7 million items (but not all research papers, and not all full-texts). Below are data for just the full-text research papers with 1999-2002 creation dates.
http://oaister.umdl.umich.edu/o/oaister/