Jérôme,
Perhaps it's a stupid question but are your queries slower than
before? You didn't tell it.
IMHO, it's not a problem to have a high load if you have a lot of
users and your queries are fast (and with 8.1, they should be far
faster than before).
To take a real example, we had a problem with a quad xeon running
postgres 7.4 and even when there were a lot of queries, the load was
always lower than 4 and suddenly the queries were really slow and the
database was completely unusable.
When we upgraded to 8.1, on very high load, we had a far higher cpu
load but queries were far faster even with a high cpu load.
Considering your top output, I suspect you use HT and you should
really remove it if it's the case.
--
Guillaume