Yes, SQL 2005 can be installed on 64 bit operating systems and used with TestCaddy but it is not recommended.

On 64bit operating systems, SQL 2008 is recommended. Use our package which includes SQL2008 Express or before installing TestCaddy, download and install SQL Server 2008 Express, then install TestCaddy and point it to the installed SQL Server 2008 Express.

On the other hand, if you still wish to use SQL 2005, we have tested TestCaddy on Vista 64 bit using SQL 2005 Express without Service Pack 2 and it appears to operate without failure, however we recommend applying SQL 2005 Service Pack 2 as soon as possible after your TestCaddy installation.

When applying Service Pack 2, note that you must be careful to follow the instructions given in this FAQ, as you could end up with a 'SQLExpress' instance alongside your TestCaddy instance, rather than upgrading it.