To begin with, SQL 2005 will give you about a 15-30% performance boost over SQL 2000 without making any SQL code change.
You also get much improved XML support, and a major upgrade to DTS called SQL Server Integration Services. SSIS is a major upgrade to DTS and is much more powerful.
SQL 2005 includes database mirroring which can give you a multi-site high availability solution without the need for high end storage systems. If you need to cluster your databases Enterprise Edition is no longer required or clustering.
You also have the option for online index rebuilding where you can rebuild your indexes without locking any of the objects.
If you are thinking about SQL 2005 you might want to move to SQL 2008 instead as it was just released. Either one will have a steep learning curve, but if you are going to upgrade you may want to go with the newer product.