You still have a number of things to think about.
1. If SQL is set to 32 bit it can only access up to 4GB of memory and has to share the 4GB of memory with the OS and application. To get around this the AWE setting and Lock Pages in Memory will need to be set. http://support.microsoft.com/kb/811891
2. You may also need to set the PAE flag in the boot.ini file.
3. Do you have any re-indexing maintenance jobs created/running on your system? Check out the article: http://www.sqlservercentral.com/Forums/Topic150354-5-1.aspx
4. Have you evaluated if additional new indexes need to be created? How much do you understand about indexes needs to be determined first. The link to the video below is very informative but assumes you have a basic understanding of performance analysis and indexes to begin with. Before doing anything with this in a production environment - always try first on your test system and always ensure you have made backups!
Index Analysis: http://technet.microsoft.com/en-us/sqlserver/gg545020.aspx
About This Video: Overview of index analysis techniques, validating if your indexes are using the right combination of keys, if your indexes are being used and how, if there are missing indexes and if your indexes are still healthy and most optimal.