the impact is very minimal; only if you replace a server does it require recreating logins/jobs/etc.
typically, you'd right click on my computer>>Properties>>Network Identification and give the computer a new name. then, after it reboots, you'd do exactly as you described:
sp_dropserver 'old server name'
sp_addserver 'new server name', 'local'
you won't have to recreate any of the items you mentioned. those items are stored in the master database or msdb database, and are not referenced in any way by servername
the only real change you would see is if you did select @@servername, it would have tne new value
Instances would be handled automatically, ie if you had installed any instances of SQL2K or SQL 2005, then people accessing them would simply change from serveroldname\instancename to servernewname\instancename, still witht he same username/passwords that they were using previously.
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