If you have 64-bit Windows you will get some performance improvement by installing the 64-bit version of PostgreSQL 9.0, but I don't have any figures for the exact improvement.
Please be aware that in order to use your database(s) in a new major version of PostgreSQL you will need to make a
Backup before you update (while you are still using 8.3) and restore it when you are running 9.0.
If doing large bulk imports you may find better overall performance by splitting the import into somewhat smaller sets, and just running housekeeping once at the end.