per backend nofsync

Hi,
  I woul like to know if it's possible to set a nofsync behavior of a
pgsql database at the client connection time. The fact is that I have
- important but not frequently consulted/created/updated datas
- not important but very frequently updated datas
in my database.
Thus I would like to speed up the things for the not important datas
while remain with slower but more secure fsync behavior for my
important datas
if it's possible, please give me the exact syntax of the connection
option.
By the way, my client is PHP4.

regards,

Antoine Ameurlain
Prosodie