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