This article review all things you need to think when you plan to install a SQL server for an Sharepoint 2007/2010 infrastructure.
Check list before the installation:
_Virtual or physical server? Keep in mind that is not recommended to install SQL server for SP into a virtual machine due that SP consummes more disk i/o, storage, processor and memory.
_High avaibility? What else? It's strongly recommended to install SQL server in cluster, but now with SP 2010 you can use the SQL mirroring technologies to provide high avaibility if you do not want to use the MS cluszering technologie.
_Which version about SQL? Avoid SQL 2005 installation definitively. Focus your efforts on SQL 2008 installation. The First question is SQL 2008 or SQL 2008 R2? If you install a MOSS 2007 then choose SQL 2008 but if you plan to install SP 2010 then install SQL 2008 R2 or SQL 2008 SP1 with CU2 (this is mandatory for SP 2010). If your SQL servers have at maximum 8 CPUs, 64 Gb of memory and only two nodes in case of cluster installation choose SQL 2008/R2 Standard edition, if you have servers with more resources you must install the Enterprise edition.
_Dedicated SQL instance or not? For Sharepoint infrastructure you must have dedicated SQL instance and no other programs or applications installed on the SQL server because SP consume more disk i/o and storage. For SP 2010 it is recommended to install one SQL instance for the content databases and another instance for search databases.
_Plan your storage. Choose if it's possible a storage with great performance about disk i/o, plan to format each lun with 64K cluster size, and provision one lun for databases system + tlogs, one or many luns for SP content databases, one or many luns for tlogs about content databases. Apply the same policy for search databases, with SP 2010 it's better to have an another SQL instance to hosts the search databases. For better performance you can isolate the SP config and admin content databases on a separated and dedicated lun.
_Plan your backup. For the tool who backup your databases, I prefer to choose MS SCDPM, but if you don't have this tool, you can script a backup with the stsadm command line or by powershell cmdlet in SP 2010. In other case you can backup your databases with you backup software. To restore it, plan to have an another farm (recovery farm), that means to have sufficient storage on this farm and this farm ust have the level about templates, features, solutions, etc.
That all! This article has been write as is and provides some basics information for your governance plan about your infrastructure and to ask you the rights questions.