Rambilla,

The current released version of SQL Server is SQL Server 2008. It has one service pack, having been released in Aug 2008 and it is fine for production. You need to test your code/application, but there are plenty of people using this in production and it is what Microsoft is selling.

The next version, which I believe will be SQL v10.5, is SQL Server 2008 R2 and it is scheduled to be released next year in 2010. There is a public CTP out now, but it is not supported by Microsoft for production use.

If you want to upgrade in March, you should upgrade to SQL 2008, but I would recommend that you not do that if you have a choice. I would look for a July/Aug upgrade to SQL 2008 R2. That way you would have support for 5 full years and not be unsupported as quickly.