As indicated on the Upgrade document, you can upgrade up to version 2.3.4.5 which was the last version to support MS SQL 2000. However there was a security issue identified existing as of version 2.3.4.3 and possibly also affecting older versions including your current version. It was fixed as of version 2.3.5.3 but upgrading to that version does require migrating to a newer version of MS SQL.
I recommend reading all the release notes from your current version up to the latest version and it would be best to migrate your current version to SQL 2005/2008, then upgrade all the way to the latest version.
Best,
Joe