When we virtualized we took the opportunity to rebuild the server from Win 2003 to 2008. So we did not use the VMWare Converter. The other thing you'll need to think about is your database. If you're using Oracle, you may not be able to virtualize using ESX because of how Oracle licenses its database and how VMWare allocates resources. We ended up using OVM from Oracle to virtualize our database server. We were already doing this for our Oracle ERP so it made sense for us. If you don't already have it in house, it's definitely a different beast with less capability than ESX.
Steve D.