Building full-text index

I am trying to build a full-text index (gin(to_tsvector('english',
title || abstract))) on about 18 million abstracts and titles from
medical literature. However, I keep getting out-of-memory errors. (I
am on a 32Gb linux system with maintenance_work_mem set to 20Gb and
shared buffers at 4Gb; postgres 8.3beta). Does creation of a
full-text index require that the entire index fit into memory? I know
this is probably a naive question, but I guess I assumed that the file
system could be used. Any hints on how to build such an index?

Thanks,
Sean