UPDATE memory exhaustion

Hey,
I was having problems with UPDATE, so I looked through the archives. Back
around the 20th of May, there was a thread about update using all memory
(thread: strange behavior of UPDATE).

It now looks that I am having that same problem on pg 6.5.1.
Basically I tried running a simple query:
update av_parts set nsn = 'xxxxx' where nsn = '';

And postgres started chugging along. After a while(not sure how long) it
was using all memory on the computer.

The box has 82MB of memory and 128 MB of swap.
The query is trying to update 3.5 million rows.

I would try to gdb to the process and see where it's spending its time,
unfortunately that box is pretty much dead until I reboot it. I'll try to
do it again later with a ulimit so I can actually log into the box :)

Thanks,
Ole Gjerde