renumber id's in correct order (compact id's)

I've entries with id's like:

x | id
---+----
b | 1
a | 4
e | 5
c | 12
d | 19
(5 rows)

now I'd like to have the id in continuing number to get:

x | id
---+----
b | 1
a | 2
e | 3
c | 4
d | 5
(5 rows)

Simpliest way to do would be to create a sequence and update the whole
table using nextval on the sequencec. Unfortunately UPDATE does not know
about an order-statement.

Any Idea,
thnx,
peter

--
mag. peter pilsl
goldfisch.at
IT-management
tel +43 699 1 3574035
fae +43 699 4 3574035
pilsl(at)goldfisch(dot)at