Usually, the newer version of SQL Server the better. Mostly it's features
available. SQL Server 2005 has a lot of things different about it.
Unfortunatley, most won't mean much to you if you haven't developed for SQL
Server for a while. SQL Server 2005 is nice because there is a free express
version that you can download and use to get your feet wet with developing
for SQL server. One of  the nice features is the inclusion of a native XML
datatype within SQL Server, as well as integration with the .Net CLR. Like I
said though, a lot of these things won't mean much if you haven't been
developing with them for a while. One of the important factors though, is
determining which version your chosen web host has. If your host/company
only has SQL Server 2000 and isn't planning to upgrade anytime soon, you may
as well start with SQL Server 2000 since it's always easier to upgrade to a
newer version then it is to check if you've used features from a newer
version to go backwards from SQL 2005 to SQL 2000.