How to upgrade SQL server 2005

use SKUUPGRADE=1 swtich from the command prompt with setup.

Eg :
start /wait setup.exe ADDLOCAL=SQL_Engine INSTANCENAME=MSSQLSERVER UPGRADE=SQL_Engine SKUUPGRADE=1 /qb

Refer
http://msdn2.microsoft.com/en-us/library/ms144259.aspx#skuupgrade

Evaluation to EE Upgrade
Refer :
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2111140&SiteID=1

Troubleshooting Performance Problems in SQL Server 2005

Good resource on Troubleshooting Performance Problems in SQL Server 2005
http://www.microsoft.com/technet/prodtechnol/sql/2005/tsprfprb.mspx