My thoughts:-
1. In Microsoft Server 2003 R2, you have an option of dumping your perfmon counters into a table.
Though this would result in lots of data and would sequentially grow if you include lots of counters and objects.
2. Sql 2005 has a DMV called sys.dm_os_performance_counters which gives same data as perfmon but in this case
you have to poll this DMV regularly and store the results and analyze them later.
Lately i have been thinking of benchmarking the server performance and your idea of avg and max sounds good to me.
I have started noting the Avg Disk queue length, %processor time and Batch Requests/sec, to figure out the current usage.
Thats all i can think of right now 🙂