disk writes within a transaction

Can someone enlighten me to how postgres handles disk writing? I've read some generic remarks about buffers, but that's about it.

We have a chunk of code that calls Postgres in a less-than-optimal way within a transaction block. I'm wondering where to prioritize fixing it, as the traffic on the wire isn't an issue.

Basically the code looks like this:

begin;
update foo set foo.a='1' where foo.bar = 1;
...
update foo set foo.b='2' where foo.bar = 1;
...
update foo set foo.c='3' where foo.bar = 1;
commit;

If the updates are likely to be a memory based operation, consolidating them can wait. If they are likely to hit the disk, I should schedule refactoring this code sooner than later.