I have one problem (or may be don't), I have SunFireV880 with 8Gb of ram and 4CPUs. System now support 400-500 concurrent users (no MTS) and free memory about 100MB. System load average from 2 to 3. It's OLTP system.
How many concurrnet users now support you system?
How many memory used\free in you configurations?
What system load average do you have?
I know that memory and cpu usage very depend on application, but ...