tune postgres for UPDATE

Hi,

I have a table with a lot of columns (text and integer).

It currently has 3Mio Rows.

Updating a column in all rows (integer) takes endless (days).

The column I update is not indexed.

How can I tune postgres to do this much more quickly?

VMstat looks like this:
r b swpd free buff cache si so bi bo in cs us
sy id wa
0 1 188 14160 16080 867064 0 0 880 888 168 479 1
2 0 97
1 1 188 15288 16080 865980 0 0 832 512 152 474 7
2 0 91
0 1 188 15464 16080 865348 0 0 872 592 144 461 2
1 0 97

so mostly iowait.

iostat shows about 10000 block writes per second.

My systems is debian-lenny (postgresql 8.3.5)

I already increased checkpoint_segments to 32, shared_buffers to 200MB

I also tried do disable autovacuum

here is a sample statement:

update users set price = (select price from prices where type =
'normal_price' and currency = users.currency)

(the table price only has 30 rows)

thank you very much!
sebastian