I know that all 3 of you have been shivering with anticiPATION at how my PGA tuning experiment went. Today is your lucky day.

I did indeed raise the value of pga_aggregate_target to 512MB. The cache hit ratio rose to near 80%, but the estimated over-allocations was quickly into the tens of thousands again. Looking at V$PGA_TARGET_ADVICE, giving the PGA 1024MB (or 1GB) seemed the next logical choice. That change was made last weekend, and after one week, I'm getting nothing but love:

SQL> SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb,
       ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc,
       ESTD_OVERALLOC_COUNT
  FROM V$PGA_TARGET_ADVICE;

 TARGET_MB CACHE_HIT_PERC ESTD_OVERALLOC_COUNT
---------- -------------- --------------------
       128             60                13700
       256             64                12475
       512             82                 5614
       768             91                  143
      1024             95                    0
      1229             96                    0
      1434             96                    0
      1638             96                    0
      1843             96                    0
      2048             96                    0
      3072             96                    0
      4096             96                    0
      6144             96                    0
      8192             96                    0
I've put my current level in blue above. I'm satisfied with these numbers at 95% and 0.