It is not feasible to come with a suggestion that works best for all installations. One problem with autovacuum is that OpenACS contains several huge tables, and many small tables, so the best tuning could be probably achieved if the parameters are defined by table. Check out the script in [1] for figuring out, the consequence of the current settings and which tables might qualify for per-table settings.
Just as a reference, Learn@wu (60+ mio objects) uses the following autovacuum settings (defaults are written as comments).
[1] http://dba.stackexchange.com/questions/21068/aggressive-autovacuum-on-postgresql
Hope this helps
-g
log_autovacuum_min_duration = 1s # -1 disables, 0 logs all actions and
autovacuum_analyze_scale_factor = 0.03 # fraction of table size before analyze; default 0.1
autovacuum_vacuum_scale_factor = 0.06 # fraction of table size before vacuum: default 0.2
autovacuum_vacuum_threshold = 300 # min number of row updates before vacuum: default 50
autovacuum_analyze_threshold = 300 # min number of row updates before anaylze: default 50
